diff --git a/.gitignore b/.gitignore
index e1fd5f1f..119e22e3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,5 @@
 *.gz
 *.pyc
 __pycache__/
+
+.idea
diff --git a/ch05/common b/ch05/common
new file mode 120000
index 00000000..60d3b0a6
--- /dev/null
+++ b/ch05/common
@@ -0,0 +1 @@
+../common
\ No newline at end of file
diff --git a/ch05/dataset b/ch05/dataset
new file mode 120000
index 00000000..aa7f0465
--- /dev/null
+++ b/ch05/dataset
@@ -0,0 +1 @@
+../dataset
\ No newline at end of file
diff --git a/common/functions.py b/common/functions.py
index 0453ae6c..42f6f173 100644
--- a/common/functions.py
+++ b/common/functions.py
@@ -39,9 +39,12 @@ def softmax(x):
     return np.exp(x) / np.sum(np.exp(x))
 
 
-def mean_squared_error(y, t):
+def squared_error(y, t):
     return 0.5 * np.sum((y-t)**2)
 
+def mean_squared_error(y, t):
+    n = len(y)
+    return np.sum((y-t)**2) / n
 
 def cross_entropy_error(y, t):
     if y.ndim == 1:
diff --git a/dataset/mnist.pkl b/dataset/mnist.pkl
new file mode 100644
index 00000000..57e81d62
Binary files /dev/null and b/dataset/mnist.pkl differ