diff --git a/teyaku.c b/teyaku.c
index 58f5588..cd46ca7 100644
--- a/teyaku.c
+++ b/teyaku.c
@@ -3,7 +3,7 @@
 #include "teyaku.h"
 #include "card.h"
 
-SetTeyaku calculate_set_teyaku(Hand h) {
+SetTeyaku calculate_set_teyaku(const Hand h) {
   int month_counts[12] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
   int month_stands[12] = { 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 1 };
   for (int i = 0; i < h.count; i++) {
@@ -38,7 +38,7 @@ SetTeyaku calculate_set_teyaku(Hand h) {
   return SET_TEYAKU_NONE;
 }
 
-ChaffTeyaku calculate_chaff_teyaku(Hand h) {
+ChaffTeyaku calculate_chaff_teyaku(const Hand h) {
   int ribbons = 0;
   int animals = 0;
   int brights = 0;
@@ -67,22 +67,22 @@ static char *set_teyaku_english_array[11] = { "None", "Triplet", "Standing Tripl
 static int chaff_teyaku_points_array[6] = { 0, 2, 3, 3, 4, 4 };
 static char *chaff_teyaku_english_array[6] = { "None", "Red", "One Ribbon", "One Animal", "One Bright", "Empty Hand" };
 
-int set_teyaku_points(Hand h) { 
+int set_teyaku_points(const Hand h) { 
   return set_teyaku_points_array[calculate_set_teyaku(h)];
 }
 
-int chaff_teyaku_points(Hand h) { 
+int chaff_teyaku_points(const Hand h) { 
   return chaff_teyaku_points_array[calculate_chaff_teyaku(h)];
 }
 
-char *set_teyaku_english(Hand h) { 
+char *set_teyaku_english(const Hand h) { 
   return set_teyaku_english_array[calculate_set_teyaku(h)];
 }
 
-char *chaff_teyaku_english(Hand h) { 
+char *chaff_teyaku_english(const Hand h) { 
   return chaff_teyaku_english_array[calculate_chaff_teyaku(h)];
 }
 
-int calculate_teyaku(Hand h) {
+int calculate_teyaku(const Hand h) {
   return chaff_teyaku_points(h) + set_teyaku_points(h);
 }
diff --git a/teyaku.h b/teyaku.h
index 8728188..5b88078 100644
--- a/teyaku.h
+++ b/teyaku.h
@@ -26,12 +26,12 @@ typedef enum ChaffTeyaku {
   EMPTY_HAND,
 } ChaffTeyaku;
 
-int calculate_teyaku(Hand h);
-SetTeyaku calculate_set_teyaku(Hand h);
-ChaffTeyaku calculate_chaff_teyaku(Hand h);
-char *set_teyaku_english(Hand h);
-char *chaff_teyaku_english(Hand h);
-int set_teyaku_points(Hand h);
-int chaff_teyaku_points(Hand h);
+int calculate_teyaku(const Hand h);
+SetTeyaku calculate_set_teyaku(const Hand h);
+ChaffTeyaku calculate_chaff_teyaku(const Hand h);
+char *set_teyaku_english(const Hand h);
+char *chaff_teyaku_english(const Hand h);
+int set_teyaku_points(const Hand h);
+int chaff_teyaku_points(const Hand h);
 
 #endif