To provide a brake fluid pressure generating device and a brake system whereby normally brake fluid in a second pressurizing chamber is permitted to flow into a low pressure source, and thereby only the brake fluid delivered from a first pressurizing chamber should be stored in a stroke simulator, the stroke simulator can be miniaturized, in failure, the brake fluid is supplied to an identical brake cylinder from both of the first pressurizing chamber and the second pressurizing chamber, and thereby a pressure receiving area of a first piston can be reduced independently of a required supply amount of the brake fluid to the brake cylinder.

In failure of a hydraulic power source 12 or a hydraulic control valve device 14, a master cut valve device 42 is in the communication state and a release/close valve 86 is in the shut-off state, and the brake fluid is supplied to the identical brake cylinders 20 and 22 from both of the first and the second pressurizing chambers 60 and 62. Normally, the master cut valve device 42 is in the shut-off state, and the release/close valve 86 is in the communication state, and the brake fluid in the second pressurizing chamber 62 is permitted to flow into a reservoir 84. Therefore, the stroke simulator 46 is required to store the brake fluid delivered from the first pressurizing chamber 60 only, and the stroke simulator 46 can be miniaturized compared to the conventional one.