A hydraulic braking system, comprising: a master cylinder comprising a housing , a pressurizing piston fluid-tightly and slidably fitted in the housing , a rearward chamber provided at a rear of the pressurizing piston , and a pressure chamber provided in front of the pressurizing piston ; and a rearward-hydraulic-pressure controller connected to the rearward chamber , wherein the rearward-hydraulic-pressure controller comprises: a regulator configured to control a hydraulic pressure in an output chamber of the regulator connected to the rearward chamber to a hydraulic pressure having a magnitude related to at least one of a hydraulic pressure in an input chamber and a hydraulic pressure in the pressure chamber ; and an input hydraulic pressure controller comprising a pressure increase valve provided between the input chamber and a high pressure source , and wherein the hydraulic braking system further comprises an air detector configured to detect presence or absence of air in said hydraulic braking system, the air detector being characterized by comprising: an upstream detector configured to detect presence or absence of air in an upstream portion based on a relationship between an open time (T) and the hydraulic pressure in the output chamber , the upstream portion comprising the rearward-hydraulic-pressure controller , the open time (T) being a length of time in which the pressure increase valve is in an open state; and a downstream detector configured to detect 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 comprising a portion of the braking system located downstream of the pressure chamber of the master cylinder . a master cylinder comprising a housing , a pressurizing piston fluid-tightly and slidably fitted in the housing , a rearward chamber provided at a rear of the pressurizing piston , and a pressure chamber provided in front of the pressurizing piston ; and a rearward-hydraulic-pressure controller connected to the rearward chamber , a regulator configured to control a hydraulic pressure in an output chamber of the regulator connected to the rearward chamber to a hydraulic pressure having a magnitude related to at least one of a hydraulic pressure in an input chamber and a hydraulic pressure in the pressure chamber ; and an input hydraulic pressure controller comprising a pressure increase valve provided between the input chamber and a high pressure source , and an upstream detector configured to detect presence or absence of air in an upstream portion based on a relationship between an open time (T) and the hydraulic pressure in the output chamber , the upstream portion comprising the rearward-hydraulic-pressure controller , the open time (T) being a length of time in which the pressure increase valve is in an open state; and a downstream detector configured to detect 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 comprising a portion of the braking system located downstream of the pressure chamber of the master cylinder . <EOS>