PURPOSE: To improve ride comfortability of a vehicle by a simple configuration and with high accuracy.

CONSTITUTION: A rotation sensor 21 picks up the rotation of a wheel, and a waveform shaping circuit 23 and a wheel speed calculating circuit 24 calculate the wheel speed based on picked-up signals. A band-pass filter 25 through which signals corresponding to unsprung mass-resonance frequency are passed, takes out fluctuations of the wheel speed. When the level of the fluctuations becomes large, a threshold value comparing circuit 26, a switching control circuit 27, and a driving circuit 28 control an electric actuator 11a to change the characteristics of a suspension device constantly set to 'hard', from 'hard' to 'soft'. In this case, the fluctuations represent fine irregularities (projections) of the road surface. Thus, knock-up due to the irregularities of the road surface can be softened to improve ride comfort of a vehicle. Further, because the irregularities of the road surface are detected based on the wheel speed, there is no need for the suspension device to especially devised.