An object is to provide a master cylinder that ensures the responsivity during brake actuation, improves a release delay during brake release, ensures the responsivity during automatic braking, and quickly releases the residual pressure in the hydraulic chamber.The master cylinder includes a cylinder body having a cylinder hole, a piston slidably inserted into the cylinder hole, the piston partitioning a hydraulic chamber, a communicating passage provided in the cylinder body, the communicating passage communicating with a reserver, a relief port formed in the piston, the relief port performing communication between the communicating passage and the hydraulic chamber, and a seal member housed in a recess portion in an inner peripheral surface of the cylinder hole of the cylinder body, the piston slidably passing through the seal member, the seal member performing sealing between the inner peripheral surface of the cylinder hole and an outer peripheral surface of the piston, the communicating passage communicating with the relief port during non-actuation, the piston moving to block the communicating passage from the relief port using the seal member during actuation, in which a flow passage control member for opening and closing the relief port is provided in a part of the relief port close to the hydraulic chamber.