Provided is a disk spring which has a spring portion having a circular disk shape, the spring portion to be interposed between a pair of rotary members, for example an outer plate and a piston, being relatively movable in an axial direction to come close to and separate away from each other, wherein the spring portion has a rounded cross-section portion to be brought into contact with, for example, the outer plate, and a straight cross-section portion being continuous to the rounded cross-section portion and being formed on at least one of an inner diameter side and an outer diameter side of the spring portion so as to be allowed to be brought into contact with, for example, the piston and be compressed between both the rotary members in a close contact state.