NOVELTY - Control method of fuel cell system involves controlling start of supply of oxygen-containing gas to fuel cell stack. Hydrocarbon fuel is reformed with fuel cell stack, and hydrogen-containing reformed gas is produced. Fuel cell system is provided with oxygen-containing gas supply section which supplies oxygen-containing gas to fuel cell stack, fuel process part which supplies reformed gas to fuel cell stack, combustion part which heats catalyst to which off gas discharged from fuel cell stack as fuel is introduced and is provided in fuel process part, and ignition device.    USE - Control method of fuel cell system (claimed).    ADVANTAGE - Occurrence of corrosion in fuel cell stack can be prevented, and cathode air bleeding can be suppressed.    DETAILED DESCRIPTION - Control method of fuel cell system involves controlling start of supply of oxygen-containing gas to fuel cell stack. Hydrocarbon fuel is reformed with fuel cell stack, and hydrogen-containing reformed gas is produced. Fuel cell system is provided with oxygen-containing gas supply section which supplies oxygen-containing gas to fuel cell stack, fuel process part which supplies reformed gas to fuel cell stack, combustion part which heats catalyst to which off gas discharged from fuel cell stack as fuel is introduced and is provided in fuel process part, and ignition device which is provided in combustion part and performs ignition operation during conversion of reformed gas of fuel supplied to combustion part into off gas. Fuel supplied to combustion part is converted from reformed gas to off gas. Ignition determination of off gas in combustion part is performed. When ignition of off gas in combustion part is successful and hydrogen concentration in anode of battery stack is high, supply of oxygen-containing gas by oxygen-containing gas supply section is started.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of the starting process of fuel cell system. (Drawing includes non-English language text).    Supply of hydrocarbon fuel and air to burner combustor (S111)    Supply of reformed gas to burner combustor (S114)    Modification process (S115)    Start of supply of reformed gas to stack (S116)    Start of supply of cathode air (S118)