To enhance exhaust brake force by ensuring surging absorption ability.

This compression coil spring 100 is constituted such that a spring constant is changed in three stages accompanied with increase of a flexure amount, and is set in a state of flexing by a predetermined amount in advance due to a predetermined set load. This compression coil spring comprises: a first densely winding part 110 having a first spring constant 110k; a second densely winding part 120 having a second spring constant 120k which is slightly greater than the first spring constant 110k; and a loosely winding part 130 having a third spring constant 130k which is greater than the second spring constant 120k. Flexing is preformed till the first densely winding part 110 is firmly attached from a free length, whereby a set position is prescribed. Flexing by a first predetermined slight amount is performed by the second densely winding part 120.