A pressure booster having arranged therein drive cylinders on both sides of a boosting cylinder is provided with a pair of pilot valves that are actuated when pistons of the drive cylinders abut against the moving ends thereof. A pair of actuation valves switch the supply state of a pressure fluid to pressure chambers of the drive cylinders. When the pilot valves are actuated, the pressure fluid passes through the pilot valves and is supplied to the pair of actuation valves, and the supply state of the pressure fluid is switched.