A high tension coil spring for a bed mattress having an exposed wiring portion which is formed on the coil spring and works together with a body spring to absorb an external load. A gap A is provided on at least one of upper and lower end wiring portions 14 and 14' so as to define a space in which at least one of turns 16-5 and 16-5' of upper and lower exposed wiring portions 16 and 16' moves upward and downward while the coil spring is being compressed. Frictional noise between the upper and lower end wiring portions and the upper and lower turns 16-5 and 16-5' of upper and lower exposed wiring portions 16 and 16' is fundamentally prevented. The elasticity of the upper and lower exposed wiring portions is increased. The wiring portions move only upward and downward without being displaced or deformed in the lateral direction, thereby increasing longevity.