A stepping motor having a stationary portion, a rotary portion supported on said stationary portion, and a smoothing device including a flexible member having an inside portion and a circumferential edge portion. The flexible member is made of a soft elastic body flexible in the direction of rotation of the stepping motor, and an annular inertia member is fixed on the circumferential edge portion of said flexible member. The smoothing device is mounted on the rotary portion of the stepping motor at the inside portion of the flexible member.