A driving-force transmitting member is formed of a material having a linear expansion coefficient larger than that of a sleeve bearing, and Δx1>r1×Δt×a−R1×Δt×b is satisfied, where R1 is inner radius of the sleeve bearing, r1 is outer radius of a rotary shaft unit, Δx1 is difference between the inner radius R1 and the outer radius r1 at a reference temperature, Δt is maximum amount of temperature change of the driving-force transmitting member relative to the reference temperature, a is linear expansion coefficient of the sleeve bearing, and b is linear expansion coefficient of the driving-force transmitting member.