PURPOSE: To improve maneuvering stability by preventing a plurality of hydraulic control sections turning into a fail safe state simultaneously in a complex hydraulic circuit in which a plurality of hydraulic control sections are made complex.

CONSTITUTION: When a failure occurs in a hydraulic type suspension control section 19, a fail safe valve 1 is set to the other end position C. Then, a pilot pressure pipe 14a turns into a communication state with a return side pipe 18, and a pilot pressure pipe 14b turns into a communication state with the return side pipe 18 through a throttle 4. Therefore, operation check valves 2a and 2b close directly after the occurrence of failure, the control section 19 turning into a fail safe state. The operation check valves 3a and 3b of a four wheel steering control section 20 turn into fail safe state after failure occurs, and a closing control section 20 turns into a fail safe state after the lapse of a specified time.