Out of combinations of three pins(15) and bushes(16a-16c), a clearance between one of the bushes(16a) and the pin(15) is small, and a clearance between the other two bushes(16b,16c) and the pins(15) are large, whereby torque transmission is always performed by only one pin(15) and the bush(16a) during rotation to avoid a transfer phenomenon between the pins. Disposing the pins and the bushes at an equal interval allows a good balance of rotation to provide an easy assembly.