A brake booster includes a master cylinder which is disposed rearwardly of a booster mechanism including a power piston, or toward a brake pedal. A valve mechanism which mechanically cooperates with an input shaft to produce a fluid pressure differential across the power piston includes a first valve seat formed on a valve body defined by a shank portion of the power piston, a second valve seat formed on the input shaft, and a valve head adapted to seat upon the first and the second valve seat in a direction opposite from the direction which is utilized in a conventional brake booster, or in a direction from the front side of the power piston. The valve mechanism is supplied with a fluid pressure from the front side of the power piston, thereby simplifying the construction of a brake booster of the type described.