To provide a booster cylinder for a brake system, which is capable of preventing, even on receiving different types of working fluid inside its pressure-receiving part and pressure-intensifying part, the working fluid in either of these parts from sticking to a seal member in the other part during its assembly or use.

A booster cylinder 7 comprises a pressure-receiving part 18 for receiving pressure from a pump 12 that supplies pressure fluid to a hydraulic booster 2, and a pressure-intensifying part for intensifying the pressure in wheel cylinders 10a and 10b in proportion with the pressure in a master cylinder 3 when that pressure from the pump 12 fed to the pressure-receiving part gets equal to or lower than a preset pressure, both which parts are individually and watertight arranged inside a pressure-receiving body and a pressure- intensifying body to be united. This arrangement prevents the working fluid in either of these parts from sticking to a seal member in the other part.