Ausfallsicherer Kreislauf bei einer Vierradlenkung

Provision of a fail-safe circuit which is capable of allowing a sub circuit to operate with reliability when a main circuit encounters abnormal conditions even in a system with only one supply source of pressure oil. A plurality of solenoid type control valves (3, 6) connected to an actuator (1) in parallel have centering springs respectively and are maintained at a neutral position in normal conditions. Each control valve is provided with a supply port, a return port and two actuator ports and is configured such that in the neutral position, the supply port and the return port are communicated with each other and interrupts communication with the actuator ports. And also, while the supply port (3a) of the most upstream control valve (3) is connected to a supply source (P) and the return port (6b) of the most downstream control valve (6) is connected to a tank (T), a return port (3b) of each control valve is connected a supply port (6a) of a control valve (6) located downstream from and adjacent to the connected return port (3b).