A hydraulic braking system, comprising: a plurality of brake cylinders (6, 12) respectively provided for a plurality of wheels (2, 8) of a vehicle; a hydraulic-pressure producing device; and a slip control device provided between the hydraulic-pressure producing device; and the plurality of brake cylinders (6, 12) and configured to control a hydraulic pressure in each of at least one of the plurality of brake cylinders (6, 12) to control a slipping state of each of the plurality of wheels (2, 8), the hydraulic-pressure producing device comprising a stiffness reducer configured to reduce stiffness in an upstream portion located upstream of the slip control device such that the stiffness is less in a case where the slip control device is in an operating state than in a case where the slip control device is in a non-operating state, so that vibrations of the hydraulic pressure in the upstream portion are suppressed, characterized in that the hydraulic-pressure producing device comprises a master cylinder comprising a pressurizing piston (32, 34) fluid-tightly and slidably fitted in a housing, a front pressure chamber (40, 42) provided in front of the pressurizing piston (32, 34) and connected to the upstream portion, and a rearward chamber provided at a rear of the pressurizing piston (32, 34), and a rear-hydraulic-pressure control device (28; 250) capable of controlling a hydraulic pressure in the rearward chamber, and wherein the rear-hydraulic-pressure control device (28; 250) comprises a rearward chamber stiffness reducer configured to reduce stiffness in the rearward chamber to reduce stiffness in the front pressure chamber (40; 42), and the rearward chamber stiffness reducer is comprised in the stiffness reducer. a plurality of brake cylinders (6, 12) respectively provided for a plurality of wheels (2, 8) of a vehicle; a hydraulic-pressure producing device; and a slip control device provided between the hydraulic-pressure producing device; and the plurality of brake cylinders (6, 12) and configured to control a hydraulic pressure in each of at least one of the plurality of brake cylinders (6, 12) to control a slipping state of each of the plurality of wheels (2, 8), the hydraulic-pressure producing device comprising a stiffness reducer configured to reduce stiffness in an upstream portion located upstream of the slip control device such that the stiffness is less in a case where the slip control device is in an operating state than in a case where the slip control device is in a non-operating state, so that vibrations of the hydraulic pressure in the upstream portion are suppressed, characterized in that the hydraulic-pressure producing device comprises a master cylinder comprising a pressurizing piston (32, 34) fluid-tightly and slidably fitted in a housing, a front pressure chamber (40, 42) provided in front of the pressurizing piston (32, 34) and connected to the upstream portion, and a rearward chamber provided at a rear of the pressurizing piston (32, 34), and a rear-hydraulic-pressure control device (28; 250) capable of controlling a hydraulic pressure in the rearward chamber, and wherein the rear-hydraulic-pressure control device (28; 250) comprises a rearward chamber stiffness reducer configured to reduce stiffness in the rearward chamber to reduce stiffness in the front pressure chamber (40; 42), and the rearward chamber stiffness reducer is comprised in the stiffness reducer.