diff --git a/src/dump.c b/src/dump.c
index 564c6888e6..0e993322b6 100644
--- a/src/dump.c
+++ b/src/dump.c
@@ -514,14 +514,11 @@ static size_t
 get_filename_table_size(mrb_state *mrb, mrb_irep *irep, mrb_sym **fp, uint16_t *lp)
 {
   mrb_sym *filenames = *fp;
-  uint16_t tsize = 0;
   uint32_t file_i;
   size_t size = 0;
   mrb_irep_debug_info *di = irep->debug_info;
 
-  if (lp == NULL) {
-    lp = &tsize;
-  }
+  mrb_assert(lp);
   for (file_i = 0; file_i < di->flen; ++file_i) {
     mrb_irep_debug_info_file *file;
     mrb_int filename_len;
@@ -618,50 +615,15 @@ write_debug_record(mrb_state *mrb, mrb_irep *irep, uint8_t *bin, mrb_sym const*
   return size;
 }
 
-static size_t
-write_filename_table(mrb_state *mrb, mrb_irep *irep, uint8_t **cp, mrb_sym **fp, uint16_t *lp)
-{
-  uint8_t *cur = *cp;
-  mrb_sym *filenames = *fp;
-  uint32_t file_i;
-  uint16_t fn_len;
-  size_t size = 0;
-  mrb_irep_debug_info *debug_info = irep->debug_info;
-
-  for (file_i = 0; file_i < debug_info->flen; ++file_i) {
-    mrb_irep_debug_info_file *file = debug_info->files[file_i];
-    if (find_filename_index(filenames, *lp, file->filename_sym) != -1) continue;
-
-    /* register filename */
-    *lp += 1;
-    *fp = filenames = (mrb_sym*)mrb_realloc(mrb, filenames, sizeof(mrb_sym) * (*lp));
-    filenames[*lp - 1] = file->filename_sym;
-
-    /* filename */
-    fn_len = (uint16_t)strlen(file->filename);
-    cur += uint16_to_bin(fn_len, cur);
-    memcpy(cur, file->filename, fn_len);
-    cur += fn_len;
-
-    size += sizeof(uint16_t) + fn_len;
-  }
-  for (file_i=0; file_i<irep->rlen; file_i++) {
-    size += write_filename_table(mrb, irep->reps[file_i], &cur, fp, lp);
-  }
-  *cp = cur;
-  return size;
-}
-
 static int
-write_section_debug(mrb_state *mrb, mrb_irep *irep, uint8_t *cur)
+write_section_debug(mrb_state *mrb, mrb_irep *irep, uint8_t *cur, mrb_sym const *filenames, uint16_t filenames_len)
 {
   size_t section_size = 0;
   const uint8_t *bin = cur;
   struct rite_section_debug_header *header;
-  mrb_sym *filenames;
-  uint16_t filenames_len = 0;
-  uint8_t *filenames_len_out;
   size_t dlen;
+  uint16_t i;
+  char const *sym; mrb_int sym_len;
 
   if (mrb == NULL || cur == NULL) {
     return MRB_DUMP_INVALID_ARGUMENT;
@@ -672,12 +634,16 @@ write_section_debug(mrb_state *mrb, mrb_irep *irep, uint8_t *cur)
   section_size += sizeof(struct rite_section_debug_header);
 
   /* filename table */
-  filenames = (mrb_sym *)mrb_malloc(mrb, sizeof(mrb_sym) * 1);
-  filenames_len_out = cur;
-  cur += sizeof(uint16_t);
+  cur += uint16_to_bin(filenames_len, cur);
   section_size += sizeof(uint16_t);
-  section_size += write_filename_table(mrb, irep, &cur, &filenames, &filenames_len);
-  uint16_to_bin(filenames_len, filenames_len_out);
+  for (i = 0; i < filenames_len; ++i) {
+    sym = mrb_sym2name_len(mrb, filenames[i], &sym_len);
+    mrb_assert(sym);
+    cur += uint16_to_bin(sym_len, cur);
+    memcpy(cur, sym, sym_len);
+    cur += sym_len;
+    section_size += sizeof(uint16_t) + sym_len;
+  }
 
   /* debug records */
   dlen = write_debug_record(mrb, irep, cur, filenames, filenames_len);
@@ -687,8 +653,6 @@ write_section_debug(mrb_state *mrb, mrb_irep *irep, uint8_t *cur)
   mrb_assert(section_size <= INT32_MAX);
   uint32_to_bin(section_size, header->section_size);
 
-  mrb_free(mrb, filenames);
-
   return MRB_DUMP_OK;
 }
 
@@ -890,6 +854,7 @@ mrb_dump_irep(mrb_state *mrb, mrb_irep *irep, int debug_info, uint8_t **bin, siz
   uint8_t *cur = NULL;
   mrb_bool const debug_info_defined = is_debug_info_defined(irep), lv_defined = is_lv_defined(irep);
   mrb_sym *lv_syms = NULL; uint32_t lv_syms_len = 0;
+  mrb_sym *filenames = NULL; uint16_t filenames_len = 0;
 
   if (mrb == NULL) {
     *bin = NULL;
@@ -902,16 +867,13 @@ mrb_dump_irep(mrb_state *mrb, mrb_irep *irep, int debug_info, uint8_t **bin, siz
   /* DEBUG section size */
   if (debug_info) {
     if (debug_info_defined) {
-      mrb_sym *filenames;
-
       section_lineno_size += sizeof(struct rite_section_debug_header);
       /* filename table */
       filenames = (mrb_sym*)mrb_malloc(mrb, sizeof(mrb_sym) + 1);
 
       /* filename table size */
       section_lineno_size += sizeof(uint16_t);
-      section_lineno_size += get_filename_table_size(mrb, irep, &filenames, NULL);
-      mrb_free(mrb, filenames);
+      section_lineno_size += get_filename_table_size(mrb, irep, &filenames, &filenames_len);
 
       section_lineno_size += get_debug_record_size(mrb, irep);
     }
@@ -945,7 +907,7 @@ mrb_dump_irep(mrb_state *mrb, mrb_irep *irep, int debug_info, uint8_t **bin, siz
   /* write DEBUG section */
   if (debug_info) {
     if (debug_info_defined) {
-      result = write_section_debug(mrb, irep, cur);
+      result = write_section_debug(mrb, irep, cur, filenames, filenames_len);
     }
     else {
       result = write_section_lineno(mrb, irep, cur);
@@ -975,6 +937,9 @@ mrb_dump_irep(mrb_state *mrb, mrb_irep *irep, int debug_info, uint8_t **bin, siz
   if (lv_syms) {
     mrb_free(mrb, lv_syms);
   }
+  if (filenames) {
+    mrb_free(mrb, filenames);
+  }
   return result;
 }