HYDRAULIC BOOSTER

PROBLEM TO BE SOLVED: To provide a technique which can supply an operation liquid effectively and obtain the boost action of a booster surely by the second operation liquid supply system including an emergency pump, even when the pipe extended from a drain port is damaged. SOLUTION: A drain connection passage 1300 for connecting a drain port open/close valve inside the housing 210 of a hydraulic booster with a drain port 130 positioned on the outer wall part of the housing 210 is formed inside the housing 210. An auxiliary reservoir tank 700 which can store the operation liquid with a capacity required to a second operation liquid supply system is arranged on the way of the drain connection passage 1300. The capacity of this auxiliary reservoir tank 700 is decided based on the boost operation guaranteed by the second operation liquid supply system, for example, by about 100-150cc. It is favorable that the auxiliary reservoir tank 700 is formed by a hole inside the housing 210 of the booster, for example, a hole 710 extending an parallel with the cylinder hole 212 in which a servo piston 400 is inserted.