In a negative pressure type booster device, secondary passages 53 communicating directly with the atmosphere is formed between the internal surface of a sliding cylindrical portion 8b of a valve piston 8 and the external surface of a silencer 27, and communication means 60 is provided for being controlled to vary its path area in dependence on the advance amount of an input rod 23 when the same is moved beyond a predetermined distance relative to the valve piston 8. Thus, when a brake pedal 25 is stepped on strongly, the atmospheric air can be led from the secondary passages 53 through the communication means 60 to a variable pressure chamber 6, so that the property of the responsiveness in braking operation can be improved to a responsiveness enhancing property which varies in dependence on the manner of stepping the brake pedal 25.