To change the drive torque, without changing the rotational speed by changing the ratio of the time, when a current flows to only one phase of coil out of plural phases of coils to the time when a current flows to two phases of coils, according to the load working on an actuator, by means of a control unit.

A shock absorber opens a valve hole into a desired aperture by the drive of a stepping motor and controls the flow of oil, sorting it to the right, when the attenuating force is large and to left when it is small. Defining that the time of one-phase application of the current let flowed to a stepping motor coil is set to Δt1 and the time of two-phase application of it is Δt2, and that the time Δt=Δt1+Δt2, the rotational speed of the stepping motor does not change as long as the Δt is fixed. Thus the drive torque of the stepping motor is changed without the change of drive frequency of the stepping motor by fixing the Δt, and changing the ratio of Δt1 to Δt2, whereby the occurrence of oil shocking sound, etc., can be suppressed.