Disclosed herein is a master cylinder. The master cylinder according to the present embodiment includes a hydraulic block provided with a main bore formed therein in an axial direction, a first piston having one side inserted into the main bore to be displaceable and the other side exposed to an outside of the hydraulic block and connected to a brake pedal, a mounting block provided with a sub bore formed therein in the axial direction and the sub bore where the first piston is inserted thereinto to be displaceable and passes therethrough and a connection port connecting the sub bore and a reservoir, a cap provided on one side of the first piston, and a gap flow path formed by a gap between the first piston and the cap so that a first liquid pressure chamber partitioned by the first piston communicates with the connection port on the main bore.