A master cylinder apparatus includes: an input piston that can be moved forward by operating a brake operating member; a pressure piston that is provided in front of the input piston to be capable of moving relative to the input piston; and a stroke velocity ratio modification device that is capable of modifying a stroke velocity ratio, which is a ratio between a stroke velocity of the pressure piston and a stroke velocity of the input piston, in at least two stages while the input piston moves from a rear end portion position to a front end portion position, and that sets the stroke velocity ratio at 1 when an abnormality occurs—in the master cylinder apparatus.