To provide an actuator capable of stably holding torque in the direction of reverse rotation from a driven member side for a long period of time.

Since a flange part 10b is provided with steps, a friction member 15 is worn due to use and is abutted on even a surface of an outer side part 10d of the flange part 10b. Consequently, since a position in the radial direction where friction force acts is displaced, Rf + Δ is realized, and braking force after use can be set to (F' x (Rf + Δ)) even if coefficient of friction is reduced to μ' (<μ) by use, it can be set substantially equally to braking force (F x Rf) in an initial condition by setting Δ to a proper value.