In a vacuum type brake booster, the reaction of output transmitted from an output shaft to a reaction member is transmitted to a valve body, a plunger, and a stepped pin. A valve mechanism, which is assembled to an axial hole of the valve body, includes an atmospheric valve seat which advances and retreats together with the plunger, a vacuum valve seat assembled to the valve body to advance and retreat, and a valve member having an atmospheric valve portion which forms an atmospheric valve in cooperation with the atmospheric valve seat and a vacuum valve portion which forms a vacuum valve in cooperation with the vacuum valve seat. The stepped pin is pushed rearward when a corresponding portion of the rear surface of the reaction member deforms and bulges rearward, and pushes the vacuum valve seat rearward in relation to the valve body.