An improved brake hydraulic pressure generating device is provided in which the hydraulic pressure supplied from a hydraulic pressure source is adjusted to a value corresponding to the brake operating force by means of a pressure adjusting valve which is activated according to displacement of an input piston or a simulator piston, and while the device is normal, a master cylinder is activated by the output hydraulic pressure. If the hydraulic pressure source or a hydraulic line connecting thereto fails, a sufficient braking force is ensured with a short stroke. There are provided a shutoff valve which shuts off communication between the simulator chamber and the atmospheric reservoir when the stroke of the input piston has exceeded a predetermined value, and a check valve which permits the flow of fluid from the atmospheric reservoir to the simulator chamber. This suppresses an unavailable stroke of the simulator piston without impairing the return of the simulator piston.