This coil spring has a part of high hardness and a part of lower hardness than the part. Also, it is preferable that the hardness of the part that comes into contact with other members is lower than the hardness of the part other than the part that comes into contact with the other members. Moreover, it is preferable that the hardness of an end turn part of the coil spring is lower than the hardness of the part other than the end turn part. Furthermore, it is preferable that the hardness of the part that comes into contact with a receiving member adapted to receive the end turn part of the coil spring is lower than the hardness of the part other than the part that comes into contact with the receiving member.