The fuel cell system is provided with the hydrogen gas circulation system that supplies hydrogen gas to the fuel cell while merging the hydrogen gas discharged from the fuel cell with the newly supplied hydrogen gas, a hydrogen gas supply valve that controls the amount of hydrogen gas supplied to the hydrogen gas circulation system, an initial pressurization unit that pressurizes the hydrogen gas circulation system by temporarily opening the hydrogen gas supply valve at the start of the fuel cell, and a re-pressurization and gas leak detection unit that re-pressurizes the hydrogen gas circulation system by opening the hydrogen gas supply valve when a given level of pressure drop is found in the pressure detected by the pressure detection unit after the pressurization by the initial pressurization unit and detects a hydrogen gas leak from the hydrogen gas circulation system based on the pressure detected by the pressure detection unit in a given timing after the pressurization.