In one embodiment, a pair of spring seats (32) are intended for use 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 a respective end of the small coil spring and a contacting surface (36). The contacting surface (36) is formed with slanting portions (37, 38) with V-shaped cross section, mutually perpendicular to one another. The power input rotary member and the power output rotary member contact a respective slanting section of one of the slanting portions (37, 38). In another embodiment, a spring seat (40) is provided with a first support portion (43) and a second support portion (44) such that the orientation of the spring seat (40) determines a phase or orientation of the power input rotary member with respect to the power output rotary member. In a third embodiment, a friction element is formed of resin and includes a friction adjusting element (20). The body portion (15a) of the friction element is made of resin and includes a friction generating surface contactable to the side surface of a sub-plate (5). The fiction adjusting element (20) is provided in the friction generating surface of the body portion (15a) by molding and is intended for modifying the friction coefficient thereof.