A master cylinder device (110; 500; 700; 782) for supplying a pressurized brake fluid to a brake device provided in a wheel and actuated by a pressure of the brake fluid, comprising: a housing (300; 510; 702) whose front side is closed and which has a separation wall (318; 721) for separating the inside of the housing into a front side chamber (326; 723) and a rear side chamber (327; 724); a pressurizing piston (302; 704) disposed in the front side chamber of the housing such that a pressurizing chamber for pressurizing the brake fluid to be supplied to the brake device is defined in front of the pressurizing piston; an input piston (306; 708) which is disposed in the rear side chamber of the housing, whose rear end portion is connected to a brake operation member, and which moves forward by an operation force applied to the brake operation member; an operation-reaction-force applying mechanism for applying an operation reaction force according to a forward movement amount of the input piston to the input piston; wherein an input chamber into which a brake fluid from a high pressure source is introduced is defined in the rear of the pressurizing piston; a transmission rod (340; 726) which is through the separation wall, whose proximal end portion is fixed in one of the input piston (306; 708) and the pressurizing piston (302; 704), and whose distal end portion (343; 729) is apart from the other of the input piston and the pressurizing piston (302; 704) in a state in which the input piston (306; 708) is not moved forward, wherein the master cylinder device (110; 500; 700; 782) is configured such that, in a state in which the distal end portion of the transmission rod does not abut on the other of the input piston and the pressurizing piston (302; 704), the brake fluid in the pressurizing chamber is pressurized depending on a force caused by a pressure of the high pressure source, characterized in that, in a state in which the distal end portion of the transmission rod abuts on the other of the input piston (306; 708) and the pressurizing piston (302; 704), the brake fluid in the pressurizing chamber is pressurized depending on a force in which the operation force is added to the force caused by the pressure of the high pressure source. a housing (300; 510; 702) whose front side is closed and which has a separation wall (318; 721) for separating the inside of the housing into a front side chamber (326; 723) and a rear side chamber (327; 724); a pressurizing piston (302; 704) disposed in the front side chamber of the housing such that a pressurizing chamber for pressurizing the brake fluid to be supplied to the brake device is defined in front of the pressurizing piston; an input piston (306; 708) which is disposed in the rear side chamber of the housing, whose rear end portion is connected to a brake operation member, and which moves forward by an operation force applied to the brake operation member; an operation-reaction-force applying mechanism for applying an operation reaction force according to a forward movement amount of the input piston to the input piston; wherein an input chamber into which a brake fluid from a high pressure source is introduced is defined in the rear of the pressurizing piston; a transmission rod (340; 726) which is through the separation wall, whose proximal end portion is fixed in one of the input piston (306; 708) and the pressurizing piston (302; 704), and whose distal end portion (343; 729) is apart from the other of the input piston and the pressurizing piston (302; 704) in a state in which the input piston (306; 708) is not moved forward, wherein the master cylinder device (110; 500; 700; 782) is configured such that, in a state in which the distal end portion of the transmission rod does not abut on the other of the input piston and the pressurizing piston (302; 704), the brake fluid in the pressurizing chamber is pressurized depending on a force caused by a pressure of the high pressure source, characterized in that, in a state in which the distal end portion of the transmission rod abuts on the other of the input piston (306; 708) and the pressurizing piston (302; 704), the brake fluid in the pressurizing chamber is pressurized depending on a force in which the operation force is added to the force caused by the pressure of the high pressure source.