To provide a simple master cylinder of a constitution for performing a first fill.

The master cylinder 1 forms a large-diameter pre-load chamber 10 and a first small-diameter hydraulic chamber 9 with a stepped cylinder 3 and a primary piston 7 slid on the stepped cylinder 3. In addition, the large-diameter pre-load chamber 10 and the first small-diameter hydraulic chamber 9 are divided to provide a third cup seal allowing the flow of a brake liquid from a large diameter hydraulic chamber to the first small-diameter hydraulic chamber on a small diameter cylinder. The large-diameter pre-load chamber 10 and an atmosphere side are divided to provide a fourth cup seal allowing the flow of a brake liquid from the large-diameter hydraulic chamber to the atmosphere side on a large-diameter piston part 7b. The brake liquid of the large-diameter pre-load chamber 10 is sent to the first small-diameter hydraulic chamber 9 by accompanying the advance of the primary piston 7. The primary piston 7 is advanced further, and the brake liquid of the inside of the large-diameter pre-load chamber 10 is flowed to the side of a reservoir 2 when the brake liquid pressure of the inside of the large-diameter pre-load chamber 10 is an open pressure or more of the fourth cup seal 20.