diff --git a/compiler/CMakeLists.txt b/compiler/CMakeLists.txt
index 5202c8b..384c8ed 100644
--- a/compiler/CMakeLists.txt
+++ b/compiler/CMakeLists.txt
@@ -6,5 +6,5 @@ add_custom_command(
         OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/y.tab.c
         COMMAND yacc -d -o ${CMAKE_CURRENT_SOURCE_DIR}/y.tab.c ${CMAKE_CURRENT_SOURCE_DIR}/grammar.y
 )
-add_library(COMPILER lex.yy.c y.tab.c codegen.c disassemble.c create.c error.c fix_tree.c opcode.c string.c compiler.c)
+add_library(COMPILER lex.yy.c y.tab.c codegen.c disassemble.c create.c error.c fix_tree.c opcode.c string.c compiler.c util.c)
 target_include_directories(COMPILER INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
diff --git a/compiler/create.c b/compiler/create.c
index 4088e87..7346e95 100644
--- a/compiler/create.c
+++ b/compiler/create.c
@@ -1,5 +1,6 @@
 #include <stdio.h>
 #include <stdlib.h>
+#include "error.h"
 #include "summoner.h"
 
 static int current_line_number()
@@ -280,6 +281,14 @@ ParameterList *chain_parameter(ParameterList *list, ParameterList *parameter)
 
 FuncDefinition *alloc_func_definition(char *name, ParameterList *parameters, TypeSpecifier *return_type, Block *block)
 {
+    if (search_function(name) || search_declaration(name, NULL))
+    {
+        compile_error(get_current_compiler()->current_line_number,
+                          FUNCTION_MULTIPLE_DEFINE_ERR,
+                          STRING_MESSAGE_ARGUMENT, "name", name,
+                          MESSAGE_ARGUMENT_END);
+    }
+
     FuncDefinition *func_d = (FuncDefinition *)malloc(sizeof(FuncDefinition));
     func_d->name = name;
     func_d->parameters = parameters;
diff --git a/compiler/fix_tree.c b/compiler/fix_tree.c
index bbe6a86..9e0ccc1 100644
--- a/compiler/fix_tree.c
+++ b/compiler/fix_tree.c
@@ -9,52 +9,6 @@ extern BuiltinFun *search_builtin_function(char *name);
 static Expression *fix_expression(Block *current_block, Expression *expr);
 static void fix_statement_list(Block *current_block, StatementList *list, FuncDefinition *fd);
 
-Declaration *search_declaration_in_current_block(char *identifier, Block *block)
-{
-    if (!block)
-    {
-        return NULL;
-    }
-
-    Declaration *d_pos;
-    for (d_pos = block->declaration_list; d_pos; d_pos = d_pos->next)
-    {
-        if (!strcmp(identifier, d_pos->name))
-        {
-            return d_pos;
-        }
-    }
-    return NULL;
-}
-
-Declaration *search_declaration(char *identifier, Block *block)
-{
-    Block *b_pos;
-    Declaration *d_pos;
-    Compiler *compiler;
-
-    for (b_pos = block; b_pos; b_pos = b_pos->outer_block)
-    {
-        for (d_pos = b_pos->declaration_list; d_pos; d_pos = d_pos->next)
-        {
-            if (!strcmp(identifier, d_pos->name))
-            {
-                return d_pos;
-            }
-        }
-    }
-
-    compiler = get_current_compiler();
-    for (d_pos = compiler->declaration_list; d_pos; d_pos = d_pos->next)
-    {
-        if (!strcmp(identifier, d_pos->name))
-        {
-            return d_pos;
-        }
-    }
-    return NULL;
-}
-
 static Expression *
 fix_identifier_expression(Block *current_block, Expression *expr)
 {
diff --git a/compiler/grammar.y b/compiler/grammar.y
index a0a6895..204cce2 100644
--- a/compiler/grammar.y
+++ b/compiler/grammar.y
@@ -145,14 +145,12 @@ return_stmt:
     ;
 
 if_stmt:
-      IF bool_expr block                         { $$ = alloc_if_stmt($2, $3, NULL, NULL); }
-    | IF bool_expr block ELSE block              { $$ = alloc_if_stmt($2, $3, NULL, $5); }
-    | IF bool_expr block elseif_list             { $$ = alloc_if_stmt($2, $3, $4, NULL); }
+      IF bool_expr block elseif_list             { $$ = alloc_if_stmt($2, $3, $4, NULL); }
     | IF bool_expr block elseif_list ELSE block  { $$ = alloc_if_stmt($2, $3, $4, $6); }
     ;
 
 elseif_list:
-      elseif
+                          { $$ = NULL; }
     | elseif_list elseif  { $$ = chain_else_if_list($1, $2); }
     ;
 
diff --git a/compiler/summoner.h b/compiler/summoner.h
index 9d510d9..b0eb960 100644
--- a/compiler/summoner.h
+++ b/compiler/summoner.h
@@ -370,4 +370,9 @@ SVM_Executable *smc_code_gen(Compiler *compiler);
 int svm_dump_instruction(FILE *fp, SVM_Byte *code, int index);
 void svm_disassemble(SVM_Executable *exe);
 
+/** util.c */
+Declaration *search_declaration(char *identifier, Block *block);
+Declaration *search_declaration_in_current_block(char *identifier, Block *block);
+FuncDefinition *search_function(char *name);
+
 #endif
diff --git a/compiler/util.c b/compiler/util.c
new file mode 100644
index 0000000..1ea10ff
--- /dev/null
+++ b/compiler/util.c
@@ -0,0 +1,62 @@
+#include <string.h>
+#include "summoner.h"
+
+Declaration *search_declaration_in_current_block(char *identifier, Block *block)
+{
+    if (!block)
+    {
+        return NULL;
+    }
+
+    Declaration *d_pos;
+    for (d_pos = block->declaration_list; d_pos; d_pos = d_pos->next)
+    {
+        if (!strcmp(identifier, d_pos->name))
+        {
+            return d_pos;
+        }
+    }
+    return NULL;
+}
+
+Declaration *search_declaration(char *identifier, Block *block)
+{
+    Block *b_pos;
+    Declaration *d_pos;
+    Compiler *compiler;
+
+    for (b_pos = block; b_pos; b_pos = b_pos->outer_block)
+    {
+        for (d_pos = b_pos->declaration_list; d_pos; d_pos = d_pos->next)
+        {
+            if (!strcmp(identifier, d_pos->name))
+            {
+                return d_pos;
+            }
+        }
+    }
+
+    compiler = get_current_compiler();
+    for (d_pos = compiler->declaration_list; d_pos; d_pos = d_pos->next)
+    {
+        if (!strcmp(identifier, d_pos->name))
+        {
+            return d_pos;
+        }
+    }
+    return NULL;
+}
+
+FuncDefinition *search_function(char *name)
+{
+    Compiler *compiler;
+    FuncDefinition *func_pos;
+
+    compiler = get_current_compiler();
+    for (func_pos = compiler->func_definition_list; func_pos; func_pos = func_pos->next) {
+        if (!strcmp(func_pos->name, name)) {
+            return func_pos;
+        }
+    }
+    return NULL;
+}