A brake system for a vehicle installable on the vehicle, the brake system comprising a brake operation member to be operated by a driver and a hydraulic brake device configured to generate a braking force that depends on a pressure of a working fluid in accordance with an operation of the brake operation member, wherein the hydraulic brake device includes two systems respectively corresponding to a right wheel and a left wheel, wherein the hydraulic brake device includes: a pair of wheel brakes which are provided respectively for the right wheel and the left wheel and which are configured to generate the braking force in accordance with a pressure of the working fluid supplied thereto; a master cylinder configured to pressurize the working fluid by a force applied to the brake operation member by the driver; a pair of master fluid passages through which the working fluid pressurized by the master cylinder is supplied to the respective wheel brakes; a pair of master shut-off valves which are provided respectively for the master fluid passages so as to shut off flows of the working fluid through the respective master fluid passages; and a controlled-pressure supply device including a high-pressure source and configured to supply, to the wheel brakes, the working fluid from the high-pressure source while controlling the pressure of the working fluid, characterized in that: in a normal condition, the master shut-off valves shut off the flows of the working fluid through the respective master fluid passages and the hydraulic brake device generates, for each of the right wheel and the left wheel, the braking force that depends on the working fluid supplied by the controlled-pressure supply device, and wherein, in a failure condition of one of the two systems, one of the following two operation modes of the hydraulic brake device is selectively established: (a) a two-wheel mode in which the master shut-off valves allow the flows of the working fluid through the respective master fluid passages and the hydraulic brake device generates, for each of the right wheel and the left wheel, the braking force that depends on a pressure of the working fluid supplied from the master cylinder; and (b) a one-wheel mode in which one of the master shut-off valves that corresponds to the other of the two systems shuts off the flow of the working fluid through a corresponding one of the master fluid passages and the hydraulic brake device generates the braking force that depends on the working fluid supplied by the controlled-pressure supply device for only one of the right wheel and the left wheel that corresponds to the other of the two systems. in a normal condition, the master shut-off valves shut off the flows of the working fluid through the respective master fluid passages and the hydraulic brake device generates, for each of the right wheel and the left wheel, the braking force that depends on the working fluid supplied by the controlled-pressure supply device, and wherein, in a failure condition of one of the two systems, one of the following two operation modes of the hydraulic brake device is selectively established: (a) a two-wheel mode in which the master shut-off valves allow the flows of the working fluid through the respective master fluid passages and the hydraulic brake device generates, for each of the right wheel and the left wheel, the braking force that depends on a pressure of the working fluid supplied from the master cylinder; and (b) a one-wheel mode in which one of the master shut-off valves that corresponds to the other of the two systems shuts off the flow of the working fluid through a corresponding one of the master fluid passages and the hydraulic brake device generates the braking force that depends on the working fluid supplied by the controlled-pressure supply device for only one of the right wheel and the left wheel that corresponds to the other of the two systems.