An operation quantity detector detects an operation quantity. Hydraulic passage systems provide connection between hydraulic chambers of a tandem type master cylinder and wheel cylinders. A hydraulic pressure source in the hydraulic passages can pressurize the hydraulic passages based on the operation quantity. Cutoff valves each is installed in each of the hydraulic passages, between the master cylinder and the hydraulic pressure source. The hydraulic pressure detectors detect hydraulic pressures of the hydraulic passages. In the opening command state, a first mutual diagnosis based on the hydraulic pressures and the operation quantity and a second mutual diagnosis based on the hydraulic pressures are performed. In the closing command state, the first and second mutual diagnoses are performed. A failure is detected in the cutoff valves and the hydraulic pressure detecting units based on results of the first and second mutual diagnoses in the opening closing command states.