NEGATIVE PRESSURE DOUBLING DEVICE

In a negative pressure booster of the present invention, a threshold line for starting a BA operation at a pedal step-in speed equal to or more than a predetermined speed is set, an inclination of the threshold line is set to a value equal to a servo ratio SR2 smaller than a servo ratio SR1 during a usual braking operation in a low input region (a small pedal step-in region) and is set to a value equal to the servo ratio SR1 during the usual braking operation in a high input region (a large pedal step-in region). That is, the threshold line for starting the BA operation is adjustable between the high input time and the low input time. Further, even when a brake pedal is stepped in at the step-in speed equal to or more than the predetermined speed in spite of a fact that the time when the braking is performed is not an emergency time, the BA operation is not performed and hence, an undesired BA operation can be prevented. In emergency and, at the same time, when the brake pedal is stepped in at the step-in speed equal to or more than the predetermined speed and with a step-in force equal to or more than a predetermined step-in force, the BA operation is started.