PURPOSE: To improve safety by detecting car height sequentially and deciding to be coarse road from the frequency where the difference against the time sequential average level will exceed over setting level while performing rolling decision from positive/negative bias of said difference and cancelling the coarse road decision under rolling state.

CONSTITUTION: A transmission timing signal (A) is provided from the output ports P3, P4 of microcomputer 8 to supersonic car height detectors 11, 12 which provide sequential car height signals to input ports P1, P2 while port 5 will provide a driving signal to operate front and rear valves 13, 14 through a drive circuit 18 thus to adjust car height. If the frquency where the detected car height and time sequential average value exceed over allowable width exceeds over predetermined level, a microcomputer 8 will decide it to be coarse road. If positive/negative interval of difference exceeding over allowable width is shifting to one way, it is decided to be rolling to cancell decision of coarse road and to correct the car height. Consequently astable operation due to erroneous decision can be prevented.