PURPOSE: To inform an abnormality of a pump at once, by providing a passage communicated at one end with the inside of the pump and at the other end with the outside of the pump, and by connecting a pressure sensor to one end of a communicating pipe having the other end connected to the opening of the passage on the outside of the pump.

CONSTITUTION: One end of a passage 13 provided in the vicinity of a check valve 6 is communicated with the inside of a pump, and the other end is communicated with the outside of the pump. A communicating pipe 14 which is connected at one end to the opening of the passage 13 on the outside of the pump, is connected at the other end with a pressure sensor 12 including an alarm circuit 15. With this arrangement the pressure in the pump acts upon the pressure sensor 13 without the use of a check valve 6 therebetween, and therefore, the pressure sensor 12 may detect an abnormality when the pump itself is ceased due to, for example, that a belt 10 is broken off, so that vacuum in the pump is eliminated. With this arrangement an abnormality of the pump itself may be detected without the operator depressing a brake pedal.