Master cylinder

A large-diameter bore portion is formed in a portion defining a pressure chamber in a cylinder body on a side opposite to a circumferential groove relative to a slidable inner-diameter portion. A communication groove capable of allowing communication between the circumferential groove and the large-diameter bore portion is formed in an upper portion of the slidable inner-diameter portion. A point of maximum depth of the communication groove is located radially outward of a bottom of the circumferential groove. The master cylinder of the present invention is capable of preventing the supply of a brake fluid from being restricted in an evacuation process, and enables a reduction in length to be machined of the communication groove, thus reducing the time for machining and suppressing an increase in cost.