PURPOSE: To sense the velocity at the time the stroke amount decreases rapidly with high precision by determining the stroke velocity VNEW from K.ΔX0/(n.Δt) one by one in compliance with increase of (n) when the stroke amountΔX is maintained below the specified level ΔX0.

CONSTITUTION: A potentiometer 26 senses the stroke position of a damper 18 at certain intervals Δt. When the stroke amount ΔX is maintained for example below the specified level ΔX0 corresponding to the min. sensing amount, the stroke velocity VNEW is determined from K.ΔX0/(n.Δt) one by one in compliance with increase of (n) under the assumption that the time when ΔX becomes below ΔX0 is n=1. Thereby the velocity when the stroke amount ΔX decreases rapidly can be sensed with high precision.