PURPOSE: To make a change of damping force unprominent even in case the expanding/shrinking motions of a damper are gentle by using the mean value as a stroke velocity when the stroke velocity is smaller than a certain specified level, wherein the mean value is such a one as determined from stroke velocities at a plurality of times preceding this applicable time of point.

CONSTITUTION: A potentiometer 26 senses the stroke position of a damper 18 at certain intervals t. A control device 34 determines the stroke velocity V on the basis of a sensing signal given by a potentiometer 26. When the stroke velocity V is smaller than a certain specified level Vo, the mean value is calculated for stroke velocities at a plurality of times preceding to the applicable point of time, and this is used as the stroke velocity. This can make a change in the damping force unprominent even in case the expanding/shrinking motions of the damper 18 are gentle because of running on a flat road.