diff --git a/README.md b/README.md
index fb22173..e2e0532 100644
--- a/README.md
+++ b/README.md
@@ -21,10 +21,11 @@ $ make execute_all
 
 
 # Benchmark build optimization flags
-- `dgemm` と `dgemv` はデフォルトで `-O3` を使用します。
-- 学習目的で最適化を無効化する場合は `OPT_LEVEL=-O0` を明示してください。
+- すべての実験ディレクトリの `Makefile` で、GCC最適化オプションはデフォルトで `-O3` です。
+- 環境変数 `OPT_LEVEL` で上書きできます（例: `-O0`, `-O2`, `-Ofast`）。
 
 ```sh
 $ make -C dgemm clean execute_all OPT_LEVEL=-O0
-$ make -C dgemv clean execute_all OPT_LEVEL=-O0
+$ make -C mandelbrot_simd clean run OPT_LEVEL=-O2
+$ make -C kdtree_nearest_neighbor clean run OPT_LEVEL=-Ofast
 ```
diff --git a/distance_computation_simd/Makefile b/distance_computation_simd/Makefile
index ad9d170..1bd6086 100644
--- a/distance_computation_simd/Makefile
+++ b/distance_computation_simd/Makefile
@@ -1,5 +1,6 @@
-CC=gcc
-CFLAGS=-O3 -std=c11 -Wall -Wextra -march=native -mavx2 -mfma
+CC ?= gcc
+OPT_LEVEL ?= -O3
+CFLAGS ?= $(OPT_LEVEL) -std=c11 -Wall -Wextra -march=native -mavx2 -mfma
 
 .PHONY: all run clean
 
diff --git a/gemm_cache_optimization/Makefile b/gemm_cache_optimization/Makefile
index cff3ca7..b64e6b0 100644
--- a/gemm_cache_optimization/Makefile
+++ b/gemm_cache_optimization/Makefile
@@ -1,5 +1,6 @@
 CC ?= cc
-CFLAGS ?= -O3 -march=native -std=c11
+OPT_LEVEL ?= -O3
+CFLAGS ?= $(OPT_LEVEL) -march=native -std=c11
 LDFLAGS ?=
 
 all: naive optimized
diff --git a/image_processing_simd/Makefile b/image_processing_simd/Makefile
index 8f7cb6a..cf05f90 100644
--- a/image_processing_simd/Makefile
+++ b/image_processing_simd/Makefile
@@ -1,5 +1,6 @@
-CC=gcc
-CFLAGS=-O3 -std=c11 -Wall -Wextra -march=native -mavx2
+CC ?= gcc
+OPT_LEVEL ?= -O3
+CFLAGS ?= $(OPT_LEVEL) -std=c11 -Wall -Wextra -march=native -mavx2
 
 .PHONY: all run clean
 
diff --git a/kdtree_nearest_neighbor/Makefile b/kdtree_nearest_neighbor/Makefile
index 0993a4c..f5cc1ad 100644
--- a/kdtree_nearest_neighbor/Makefile
+++ b/kdtree_nearest_neighbor/Makefile
@@ -1,6 +1,7 @@
-CC := gcc
-CFLAGS := -O3 -march=native -std=c11 -Wall -Wextra -Wpedantic
-LDFLAGS := -lm
+CC ?= gcc
+OPT_LEVEL ?= -O3
+CFLAGS ?= $(OPT_LEVEL) -march=native -std=c11 -Wall -Wextra -Wpedantic
+LDFLAGS ?= -lm
 
 TARGET := benchmark
 SRC := benchmark.c
diff --git a/mandelbrot_simd/Makefile b/mandelbrot_simd/Makefile
index 95e46b8..ceafcbc 100644
--- a/mandelbrot_simd/Makefile
+++ b/mandelbrot_simd/Makefile
@@ -1,5 +1,6 @@
-CC=gcc
-CFLAGS=-O3 -std=c11 -Wall -Wextra -march=native -mavx2 -mfma
+CC ?= gcc
+OPT_LEVEL ?= -O3
+CFLAGS ?= $(OPT_LEVEL) -std=c11 -Wall -Wextra -march=native -mavx2 -mfma
 
 .PHONY: all run clean