Provided is a master cylinder apparatus with a reduced size enabled by simplifying the fluid passageway configuration. A master cylinder apparatus which includes a base member containing a fluid passageway for brake fluid and to which an operation of a brake operator is inputted is provided with: a master cylinder installed on the base member to generate a fluid pressure based on an input produced by operating the brake operator; and at least two solenoid valves attached to one side of the base member to open or close the fluid passageway. The two solenoid valves are disposed symmetrically with respect to a central axis along the axial direction of the master cylinder as viewed from a direction perpendicular to the one side of the base member.