An electronic brake system comprising: a reservoir configured to store a pressing medium therein; an integrated master cylinder configured to include a master cylinder, the master cylinder including a master chamber and a master piston provided in the master chamber to form displacement changeable by operation of a brake pedal, and a pedal simulator, the pedal simulator including a simulation chamber, a reaction force piston provided in the simulation chamber to form displacement changeable by hydraulic pressure of the pressing medium stored in the master chamber to provide a reaction force corresponding to a pedal effort of the brake pedal, and an elastic member elastically supporting the reaction force piston; a hydraulic-pressure supply device configured to generate hydraulic pressure by operating a hydraulic piston by an electrical signal that is output in response to displacement of the brake pedal; and a hydraulic control unit configured to include not only a first hydraulic circuit to control hydraulic pressure applied to two wheel cylinders, but also a second hydraulic circuit to control hydraulic pressure applied to two other wheel cylinders, wherein the integrated master cylinder includes: a simulation passage configured to connect the simulation chamber and the reservoir to each other; and a simulator valve provided in the simulation passage to control flow of a pressing medium; characterized in that the integrated master cylinder includes a cylinder body, the master cylinder and the pedal simulator being coaxially arranged in the cylinder body; and the integrated master cylinder further includes a bypass passage connected parallel to the simulator valve on the simulation passage; and a simulator check valve provided in the bypass passage so as to allow a pressing medium to flow in only one direction from the reservoir to the simulation chamber. a reservoir configured to store a pressing medium therein; an integrated master cylinder configured to include a master cylinder, the master cylinder including a master chamber and a master piston provided in the master chamber to form displacement changeable by operation of a brake pedal, and a pedal simulator, the pedal simulator including a simulation chamber, a reaction force piston provided in the simulation chamber to form displacement changeable by hydraulic pressure of the pressing medium stored in the master chamber to provide a reaction force corresponding to a pedal effort of the brake pedal, and an elastic member elastically supporting the reaction force piston; a hydraulic-pressure supply device configured to generate hydraulic pressure by operating a hydraulic piston by an electrical signal that is output in response to displacement of the brake pedal; and a hydraulic control unit configured to include not only a first hydraulic circuit to control hydraulic pressure applied to two wheel cylinders, but also a second hydraulic circuit to control hydraulic pressure applied to two other wheel cylinders, wherein the integrated master cylinder includes: a simulation passage configured to connect the simulation chamber and the reservoir to each other; and a simulator valve provided in the simulation passage to control flow of a pressing medium; characterized in that the integrated master cylinder includes a cylinder body, the master cylinder and the pedal simulator being coaxially arranged in the cylinder body; and the integrated master cylinder further includes a bypass passage connected parallel to the simulator valve on the simulation passage; and a simulator check valve provided in the bypass passage so as to allow a pressing medium to flow in only one direction from the reservoir to the simulation chamber. a simulation passage configured to connect the simulation chamber and the reservoir to each other; and a simulator valve provided in the simulation passage to control flow of a pressing medium; characterized in that the integrated master cylinder includes a cylinder body, the master cylinder and the pedal simulator being coaxially arranged in the cylinder body; and the integrated master cylinder further includes a bypass passage connected parallel to the simulator valve on the simulation passage; and a simulator check valve provided in the bypass passage so as to allow a pressing medium to flow in only one direction from the reservoir to the simulation chamber.