A hydraulic braking system includes: a master cylinder including a pressurizing piston, a rearward chamber, and a pressure chamber; and a rearward-hydraulic-pressure controller connected to the rearward chamber and including a pressure increase valve and a regulator for controlling a hydraulic pressure in an output chamber. An air detector includes: an upstream detector, including the rearward-hydraulic-pressure controller, for detecting presence or absence of air in an upstream portion based on a relationship between the hydraulic pressure in the output chamber and an open time of the pressure increase valve; and a downstream detector for detecting presence or absence of air in a downstream portion based on a relationship between a stroke of the pressurizing piston and the hydraulic pressure in the output chamber. The downstream portion includes a portion located downstream of the pressure chamber of the master cylinder.