A master cylinder device including: a housing whose front side is closed and which has a separation wall for separating the inside of the housing into a front side chamber and a rear side chamber; a pressurizing piston disposed in the front side chamber so as to define a pressurizing chamber for pressurizing the brake fluid and an input chamber into which a brake fluid from a high pressure source is introduced; an input piston disposed in the rear side chamber and moving forward by an operation force; and a transmission rod which is through the separation wall, whose proximal end portion is fixed in one of the pressurizing and input pistons, and whose distal end portion is apart from the other of the pressurizing and input pistons in a state of no forward movement.