Vehicle braking system

In a vehicle braking system, a backup piston (88) pushes a master piston (26) directly from behind when hydraulic pressure of a boosted hydraulic pressure chamber decreases. The backup piston (88) has a piston body (88a) which is slidably fitted in a casing (17), and a pusher (88b) which is slidably fitted in the casing (17) with a seal diameter smaller than seal diameters of the master piston (26) and the piston body (88a) and which extends to the front end of the piston body (88a), to push the master piston (26) from behind. An annular input chamber (96) communicated with a hydraulic power source (12) is formed between the backup piston (88) and the casing (17). When the pusher (88b) pushes the master piston (26) forward, amount of volume increased in the boosted hydraulic pressure chamber (25) is set to be substantially equal to amount of volume decreased in the input chamber. Thus, it is possible to avoid increase in the hydraulic pressure of the boosted hydraulic pressure chamber (25) during forward movement of the backup piston (88), using a simple configuration with a reduced number of parts.