PURPOSE: To improve the safety while an automobile is traveling at high speed by providing a switch to be opened when a transmission gear is set to a high speed side on a control line of a setting element to be controlled to brake a driving wheel in identifying the idling of the driving wheel.

CONSTITUTION: Two hydraulic braking pipes 3, 4 extending from a main brake cylinder 1 provided with a brake pedal 2 are connected to braking pipes 3-7 to be communicated with right and left and forward and rear brake cylinders in a diagonal brake system in a branched manner, and solenoid valves 9, 10 are interposed on the brake pipes 6, 8 to a front wheel which is a driving wheel. Electric magnets 14, 15 of the solenoid valves 9, 10 are connected to a battery connection part 16 through a safety switch 21 and a normally closed switch 20 of a valve relay 17, the valve relay 17 is deenergized when an electronic part 18 identifies the result by itself, and controls the switch 20 to be opened. The safety switch 21 controls to be opened when the transmission gear on the high speed side or the traveling stage is set.