To provide a low price high pressure/low pressure switch type hydraulic booster.

This hydraulic booster is provided with two hydraulic pressure chambers 12 and 13 which are divided on both sides of a first piston 3, a second piston 17, which is connected to a piston rod 4 moving together with the first piston 3 and has smaller area presented to pressure than that of the first piston 3, a high pressure generating chamber 10 which is divided by the second piston 17, a first switching valve 21, which selectively introduce high pressure from a pump 20 and low pressure of a tank 26 into the hydraulic pressure chambers 12 and 13 on both sides of the first piston 3, an operating pressure supply passage 25 connected to the high pressure generating chamber 10 through a check valve 5, a selector valve 27, which selectively introduce specified operating pressure and drain pressure into the operating pressure supply passage 25, and a cancel operation means 15, which forcibly opens the check valve 5 at a position where the pressure of the high pressure generating chamber 10 becomes maximum.