To perform automatic rear wheel alignment in resetting from another steering mode to a standard steering mode by moving a spool for a flow passage change valve in synchronization with the movement of a piston in a rear wheel steering cylinder to change over the flow passage change valve to neutral, first and second conditions.

A pressure oil passing through a flow passage open/close valve 3 flows through a deviated flow passage change valve 4 into a chamber 115b in a rear wheel steering cylinder 115 where a piston 116 is deviated and moves in the opposite direction where the piston 116 is deviated. When the piston 116 reaches a neutral position, the flow passage change valve 4 reaches a neutral position where the flow passage change valve 4 cuts off a flow passage 2 in accordance with a geometrical relationship among an arm member 5 joined to a tie rod 117a of the piston 116, a crank member 6 and a spool 4a, so that additional pressure oil to be supplied to the chamber 115a is cut off for stopping functioning operation upon completion of rear wheel alignment.