NOVELTY - A fuel cell system (1) comprises fuel cell (10) having fuel gas channel (11) on anode side and oxidant gas channel (12) on cathode side, for generating electricity using fuel gas and oxidant gas; diluting unit for diluting gas discharged from fuel gas channel by mixing discharged gas with dilution gas; purge valve (25) for purging gas in fuel gas channel to diluting unit; scavenging unit for scavenging fuel gas channel and oxidant gas channel; and dilution assist unit for supplying dilution assist gas to diluting unit to assist dilution in diluting unit, during scavenging.    USE - A fuel cell system for mounting on fuel cell vehicle.    ADVANTAGE - Even when purge valve is activated and gas in fuel gas channel is purged during halt of electricity generation, high concentration fuel gas (hydrogen) is prevented from exhaustion from diluting unit; even when gas in fuel gas channel is discharged during scavenging, by activating dilution assist unit to supply dilution assist gas, amount of gas for dilution in diluting unit is steadily ensured; and even when fuel cell system is used in low temperature environment e.g. below zero, the produced water in fuel cell and the diluting unit are prevented from freezing, thus ensuring immediate start up in low temperature environment. The hydrogen purging during scavenging is smoothly performed by pushing low concentration hydrogen gas out of diluting unit by dilution assist gas, thus shortening time period for scavenging.    DETAILED DESCRIPTION - A fuel cell system (1) comprises fuel cell (10) having fuel gas channel (11) on anode side and oxidant gas channel (12) on cathode side, for generating electricity using fuel gas supplied to fuel gas channel and oxidant gas supplied to oxidant gas channel; diluting unit for diluting gas discharged from fuel gas channel by mixing discharged gas with dilution gas which is supplied from oxidant gas supply unit and passed through and discharged from fuel cell, and for exhausting diluted gas to outside; purge valve (25) for purging gas in fuel gas channel to diluting unit; scavenging unit for scavenging fuel gas channel and oxidant gas channel; and dilution assist unit for supplying dilution assist gas to diluting unit through assist passage connected to diluting unit to assist dilution in diluting unit, during scavenging. An INDEPENDENT CLAIM is included for operation of fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a fuel cell system.    Fuel cell system (1)    Fuel cell (10)    Fuel gas channel (11)    Oxidant gas channel (12)    Purge valve (25)