A brake booster of stroke-enlarging type which includes a power piston moved in accordance with the difference of pressure on opposite sides thereof, a control piston having therein a control valve for controlling the pressure difference and slidably fitted in a central bore of the power piston, an anti-backdown transmission mechanism disposed between the two pistons, a reaction lever abutted at an output portion thereof on an output member and, at a pair of input portions located on opposite sides of the output portion, on the control piston and the anti-backdown transmission mechanism respectively, and a return spring backwardly biasing the power piston. The anti-backdown transmission mechanism transmits the output force of the power piston to the reaction lever and the reactionary force of the reaction lever to the power piston in an initial operation stage of the booster, and after an advancing of the power piston in relation to the control piston by a predetermined amount it transmits the reactionary force of the reaction lever not to the power piston but to the control piston. An abutting portion is disposed on the power piston for abutting on the front side of the anti-backdown transmission mechanism for retracting the same to the original position together with the power piston which is forcibly retracted by the return spring.