A pair of spring seats (32) are used at opposite ends of a small coil spring (6) disposed between a power input rotary member and a power output rotary member. Each spring seat (32) includes a receiving side surface for receiving the spring and a contacting surface (36). Each contacting surface (36) is formed with four slanting surfaces that slant inward toward a central portion of the contacting surface. The intersections of the four slanting surfaces of each spring seat define two intersecting V-shaped grooves (37, 38). The power input rotary member and the power output rotary member contact respective portions of one of the V-shaped grooves (37, 38) of each spring seat. Engagement between the rotary members and the engaged V-shaped grooves restricts movement of the spring seats (32).