PURPOSE: To make an oil tank for a booster unnecessary so that a hydraulic booster device is made compact, by providing such an arrangement that a return oil passage for discharging hydraulic pressure in a hydraulic output chamber of a booster is formed in a master cylinder so that the passage may be communicated with an oil tank for the master cylinder.

CONSTITUTION: Upon resting of a booster B, an output hydraulic chamber 24 of the booster B is communicated with an oil tank 2 for a master cylinder M through a rear port 35 and a passage 36 in a valve piston 26, a small oil chamber 32 and an output port 38 in a rear piston 72, and a hydraulic supply chamber 102 and a supply port 42. With this arrangement, since there is provided such a return passage for discharging hydraulic pressure in the hydraulic output chamber 24 into the oil tank 2 for the master cylinder M, an oil tank exclusively used for a booster device B may be eliminated so that the device may be made compact with its entirety.