A master cylinder apparatus for a BBW system is provided, which eliminates a risk of brake delay, even with the use of a plunger-type master cylinder, and which therefore is able to achieve a reduction in size of a master cylinder. The master cylinder apparatus comprises a tandem-type master cylinder connected to wheel cylinders through fail-safe valves and a stroke simulator for ensuring a desired stroke of a brake pedal upon receiving a brake fluid introduced from a fluid pressure chamber in the master cylinder. The master cylinder is arranged in the form of a plunger-type master cylinder in which cup seals are provided on an inner surface of a bore of the cylinder body. A spring force of a second return spring for a secondary piston is set to be larger than that of a first return spring for a primary piston. A retracted position of the secondary piston is limited by a stopper pin extending through an oblong hole formed in the secondary piston.