A brake booster of stroke-enlarging type which includes a casing, a power piston so disposed as to divide the interior of the casing into two chambers for being actuated by the difference of pressure in the two chambers, a control piston, containing therein a valve mechanism for controlling the difference of pressure in the two chambers and a transmission mechanism for transmitting an input of an input member to an output member, being coaxially fitted into a central bore of the power piston relatively movably thereto and a reaction lever, which is abutted at an output portion in the middle thereof by the output member, receiving output of the transmission mechanism and output of the power piston respectively as an input at a first input portion and a second input portion thereof located on either side of said output portion in the middle, whereby a greater stroke can be obtained at the output member than a stroke input at the input member. Between the control piston and the power piston a locking mechanism including a ball and a ball retainer is disposed. The ball retainer is for transmitting an output of the power piston to the reaction lever via the ball, through abutment on a first input portion of the reaction lever. After the ball has been locked between the power piston and the control piston a reactionary force which comes from the reaction lever back to the ball retainer is mostly transmitted from the ball to the control piston, not to the power piston. Therefore the power piston is not pushed back even after the peak of the assisting force given to the power piston by the pressure difference.