diff --git a/pop3proto_test.go b/pop3proto_test.go
new file mode 100644
index 0000000..65d2ae2
--- /dev/null
+++ b/pop3proto_test.go
@@ -0,0 +1,95 @@
+package pop3
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"strings"
+	"testing"
+)
+
+func TestReadLine(t *testing.T) {
+	// This function calls textproto.Reader.ReadLine only.
+	// so no need　for test.
+}
+
+func TestReadLines(t *testing.T) {
+	r := reader("line1\nline2\n.\n")
+
+	s, err := r.ReadLines()
+
+	if len(s) != 2 || s[0] != "line1" || s[1] != "line2" || err != nil {
+		t.Fatalf("%v, %v", s, err)
+	}
+
+	s, err = r.ReadLines()
+
+	if s != nil || err != io.EOF {
+		t.Fatalf("EOF: %s, %v", s, err)
+	}
+}
+
+func TestReadToPeriod(t *testing.T) {
+	r := reader("line1\nline2\n.\n")
+
+	s, err := r.ReadToPeriod()
+
+	expected := strings.Replace(`line1
+line2`, "\n", "\r\n", -1)
+
+	if s != expected || err != nil {
+		t.Fatalf("%v, %v", s, err)
+	}
+
+	s, err = r.ReadToPeriod()
+
+	if s != "" || err != io.EOF {
+		t.Fatalf("EOF: %s, %v", s, err)
+	}
+}
+
+func TestReadResponse(t *testing.T) {
+	var r *Reader
+	var s string
+	var err error
+
+	r = reader("+OK message\n")
+	s, err = r.ReadResponse()
+	if s != "message" || err != nil {
+		t.Fatalf("%v, %v", s, err)
+	}
+
+	r = reader("-ERR message\n")
+	s, err = r.ReadResponse()
+	if s != "" || err == nil || err.Error() != "message" {
+		t.Fatalf("%v, %v", s, err)
+	}
+
+	r = reader("message\n")
+	s, err = r.ReadResponse()
+	if s != "" || err == nil || err.Error() != "レスポンスのフォーマットが不正です。: message" {
+		t.Fatalf("%v, %v", s, err)
+	}
+
+	r = reader("* message\n")
+	s, err = r.ReadResponse()
+	if s != "" || err == nil || err.Error() != "レスポンスの内容が不明です。: * message" {
+		t.Fatalf("%v, %v", s, err)
+	}
+}
+
+func TestWriteLine(t *testing.T) {
+	var buf bytes.Buffer
+
+	w := NewWriter(bufio.NewWriter(&buf))
+
+	err := w.WriteLine("foo %d", 123)
+
+	if s := buf.String(); s != "foo 123\r\n" || err != nil {
+		t.Fatalf("s=%q; err=%s", s, err)
+	}
+}
+
+func reader(s string) *Reader {
+	return NewReader(bufio.NewReader(strings.NewReader(s)))
+}