To provide a tandem master cylinder capable of obtaining an excellent brake feeling irrespective of the fact that a double seal and a cup seal are used for a secondary side and a primary side, respectively.

A pressing force of a double seal 52 on a secondary side against a peripheral wall of a cylinder hole 14 is set smaller than that of a cup seal 51 on a primary side against a peripheral wall of the cylinder hole 14. For this purpose, an interference between the double seal 52 and the cylinder hole 14 is set to 0.1 to 0.4 mm or, in particular, to 0.1 to 0.35 mm, whereas the interference between the cup seal 51 and the cylinder hole 14 is set to 0.6 to 10 mm. With this setting, a piston 32 on the secondary side using the double seal 52 can be returned smoothly in the same manner as in a piston 31 on the primary side.