A coil spring for a bed mattress, At least one of contact wiring portions 16-5 and 16-5' of the upper and lower exposed wiring portions 16 and 16' which abut to the end wiring portions 14 while the upper and lower exposed wiring portion 16, 16' are being compressed has a contact surface A which is in contact with one corresponding end wiring portion of the end wiring portions 14, whereby frictional noise between the exposed wiring portion 16, 16' and the end wiring portion 14 is prevented. It is possible to consequently remove friction that is created due to contact between the exposed wiring portion and the end wiring portion while the exposed portion is being compressed, thereby preventing noise. The exposed wiring portion and the body wiring portion move only upward and downward without being displaced or deformed in the lateral direction, thereby increasing longevity.