PURPOSE: To restrain large swing of a boom that is apt to occur at start and stop in a boom-equipped working vehicle.

CONSTITUTION: At start and stop, a bypass valve 15 is opened to be operated prior to operation of a change-over valve 13. As a result, since a part of fluid supplied to an actuator 10 at start or stop is bypassed through a throttle valve 17 and the bypass valve 15, the turn of a boom 1 is started or stopped moderately. Therefore any large swing of the boom that is apt to occur at start and stop can be restrained.