A master cylinder apparatus for a BBW system is provided, which enables accurate detection of a piston stroke without substantial increase in the size of a master cylinder. A master cylinder apparatus includes a master cylinder adapted to be connected to wheel cylinders through fail-safe valves, a stroke simulator for ensuring a desired stroke of a brake pedal upon receiving a brake fluid introduced from a fluid pressure chamber in the master cylinder through a simulator passage, and an opening/closing device for opening and closing the simulator passage. A stroke sensor for detecting a stroke of a primary piston is formed by a rotation angle detector, a sensor arm having one end connected to a rotary shaft of the rotation angle detector and a sensor pin extending from the primary piston. Linear motion of the primary piston is converted to rotational motion through engagement between the sensor pin and the sensor arm and is transmitted to the rotation angle detector.