PURPOSE: To offer a master cylinder capable of preventing a sealing member from being assembled reversely as well as to improve accuracy of eccentricity of a piston to a cylinder holes while reducing machining manhours.

CONSTITUTION: Regarding a master cylinder equipped with a piston 5 having its head part 6 fitted slidably in a cylinder hole 2 formed in a cylinder body 1 and partitioning a liquid pressure chamber A, a shaft part 5A extending from the head part 6 of the piston 5 toward the outer side of the cylinder hole 2, and a cylindric guiding member 20 arranged between the shaft part 5A and the cylinder hole 2 and stopped from slipping out by a slip-out stopper 40 provided on the end of the cylinder hole 2 opening, the cylinder hole 2 bore where the guiding member 20 is arranged and the cylinder hole 2 bore where the head part 6 of the piston 5 slides are of identical diameters. A collar part 25 is provided on the outside circumference of the end of the cylinder hole 2 opening of the guiding member 20 and the collar part 25 is made to abut on a taper part 3 formed on the end of the cylinder hole 2 opening.