To provide a mounting structure of an injection-valve opening pin that allows reduction of cylinder length, has less components, is inexpensive, and allows easy mounting work, in a center-valve-type master cylinder.

A through-hole 24 for loosely passing the injection-valve opening pin 8 is formed in a primary piston 2. The opening pin 8, when loosely inserted into the through-hole 24, is formed in a length projecting from the outer peripheral surface of the primary piston 2. The outer peripheral surface of the primary piston 2 is provided with an annular support member 19, and the annular support member 19 is provided with a support hole for supporting both ends of the opening pin 8. The opening pin 8 is pressed into the support hole, and held in the axial direction of the opening pin 8. The opening pin 8 is held in a projecting state from the annular support member 19 toward a stopper member 12, and directly abuts on the stopper member 12.