A hydraulic braking system for an automotive vehicle having a power source for generating a hydraulic power pressure, a reservoir, a master cylinder, a hydraulic booster for actuating the master cylinder in response to depression of a brake pedal and a plurality of wheel brake cylinders for braking respective road wheels, which are divided into a first group of wheel brake cylinders communicating with the master cylinder and a second group of wheel brake cylinders communicating with the hydraulic booster. An auxiliary cylinder is disposed between the hydraulic booster and the second group of wheel brake cylinders. The auxiliary cylinder includes a cylinder body having a cylinder bore defined therein and a piston axially slidably disposed within the cylinder bore. The piston forms within the cylinder bore a first chamber and a second chamber, and blocks a fluid communication therebetween. The first chamber communicates with the hydraulic booster and the second chamber communicates with the second group of wheel brake cylinders to which a braking force is applied in response to operation of the hydraulic booster. Thereby, the brake fluid filled in the hydraulic booster is prevented from flowing out, even if leakage of the brake fluid occurs in the second group of wheel brake cylinders.