PURPOSE: To detect half-shortcircuit state downstream of a brake fluid pressure solenoid valve in an anti-lock system.

CONSTITUTION: The emitter and base of a switching transistor Tr1 are connected across the coil 53 of a solenoid valve via a divided voltage resistor, and the switching transistor Tr1 is made to operate, when a potential difference across the coil 53 is equal to or above the minimum operation voltage of the solenoid valve. In addition, a detection circuit 2 for collector current is provided at the collector of the switching transistor Tr1. According to the construction, an error can be properly detected, when the solenoid valve is caused to operate due to the occurrence of half-shortcircuit state at the downstream side of the coil 53. Also, a failsafe relay operation monitor circuit 3 is added between the coil 53 and a switch 52 for monitoring voltage downstream of the switch 52, thereby ensuring the operation mode of a failsafe relay to be properly monitored.