To provide a master cylinder allowing assembly of a valve device arranged in a communication part between a supply oil chamber of the master cylinder and a reservoir tank by a small number of part items and simple work.

In the master cylinder, the valve device 4 provided with a valve element 7 energized in a valve closing direction by a spring 8 and opening when a pressure of a supply oil chamber 2 side inside a cylinder body 1 is a predetermined value or more higher than a pressure inside an oil storage chamber 3 of the reservoir tank is interposed in a connecting part between the cylinder body 1 and the reservoir tank R attached above the cylinder body 1. It is characterized by that the valve device 4 is provided with a valve body 4a housed inside the oil storage chamber 3, a passage 6 formed in the valve body, and the freely opening and closing valve element 7 arranged inside the passage, the valve body 4 and the valve element 7 are held inside the oil storage chamber 3 by a reservoir tank plate 10, and the reservoir tank plate 10 is held and fixed by a reservoir tank R upper face and a diaphragm 12 mounted on the reservoir tank R.