An automatic brake booster includes a solenoid which drives a plunger, forming part of a valve mechanism, forward to switch a flow path in the valve mechanism, separately from an input shaft. A piston is mounted on the plunger, and the atmospheric pressure is introduced into an atmospheric pressure chamber disposed forwardly of the piston while a pressure which prevails in constant pressure chambers A, C is introduced into a negative pressure chamber disposed rearwardly of the piston, thus urging the piston rearward. Preferably, the piston has a pressure responsive area which is substantially equal to the pressure responsive area of an atmosphere valve seat formed on a plunger. This arrangement allows a variation in the output from the brake booster to be minimized or reduced substantially to zero if there is a variation in the magnitude of a negative pressure which is introduced into the constant pressure chambers A, C.