To sufficiently ensure the negative pressure in a master bag by changing the control of a throttle valve when the negative load accumulated in the master bag is less than a prescribed value.

An electronic control device 28 estimates the negative pressure in the master bag 10 according to the peak value Pp, duration Pt and increasing ratio ΔP of negative pressure, and subtracts the negative pressure in the master bag 10 on the basis of the frequency and time of operation of a brake pedal detected by a brake pedal switch 26. When the estimated negative pressure value in the master bag 10 is less than the prescribed value, the control of the throttle valve is changed so as to hasten the closing operation of the throttle valve 4.