A vehicle braking system includes: a hydraulic booster which applies output hydraulic pressure of a hydraulic power source to a boosted hydraulic pressure chamber after regulating the output hydraulic pressure as a control piston operates to balance reaction force generated by hydraulic pressure of the boosted hydraulic pressure chamber with brake operating input, the boosted hydraulic pressure chamber facing the rear face of a master piston of a master cylinder; and a brake stroke simulator installed between a brake operating member and the control piston. The hydraulic booster includes a backup piston which is slidably housed in a casing with its front face turned to the boosted hydraulic pressure chamber and which can directly push the master piston in response to operation of the brake operating member when the hydraulic pressure of the boosted hydraulic pressure chamber decreases. The control piston and the brake stroke simulator are contained in the backup piston. Thus, it is possible to improve the efficiency in assembling the hydraulic booster and brake stroke simulator to the casing of the master cylinder.