TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Process: The method of stopping operation is performed also for the fuel cell system comprising the hydrogen gas generating system. Purging is performed by the air when internal temperature of the hydrogen gas generating system is equal to or less than a predetermined temperature and pressure. Preferably, purging is performed using the air which has a temperature less than an internal temperature of the hydrogen gas generating system and drops with the passage of time. The method involves steam purging by supplying steam into the hydrogen gas generating system prior to supplying air, to discharge component(s) of residual flammable material, regardless of environmental condition. The steam is supplied at a pressure lower than the saturated vapor pressure. The steam pressure is determined based on a temperature and pressure inside the hydrogen gas generating system. The method involves switching from steam purging to air purging is performed based on a period of time for which the steam purging is performed. The method further involves de-pressurizing inside the hydrogen gas generating system after supplying the air. The method further involves inert gas purging by supplying inert gas into the hydrogen gas generating system until the environmental condition has no effect on the system from heat generated by a reaction between residual flammable material within the system and oxygen to discharge material. The predetermined raw material fuel is hydrocarbon compound.Preferred Composition: The hydrogen gas generating system comprises a vaporizing portion which vaporizes the raw material. Air heated to a temperature less than internal temperature of the hydrogen gas generating system with residual heat from vaporizing portion is supplied in the air purging. The control portion determines the pressure of the steam based on temperature and pressure inside the hydrogen gas generating system. The hydrogen gas generating system comprises an inert gas tank for storing inert gas.