A method of operating a fuel cell system includes step a) of providing a package accommodating a fuel cell stack and a hydrogen-rich gas supply configured to supply hydrogen-rich gas, step b) of detecting leakage of a combustible gas, comprised of the hydrogen-rich gas or a raw fuel gas, using a combustible gas detector, step c) of injecting the combustible gas intermittently into the package through a combustible gas guide pipe, and step d) of determining whether or not detection sensitivity of the combustible gas is proper based on an output value of the combustible gas detector when step c) is preformed.