NOVELTY - A fuel cell system comprises electric-power-generated fuel cell using fuel gas containing hydrogen and oxidizing agent gas containing oxygen, hydrogen generating apparatus which has modification part for producing reformed gas which carries out steam reforming of raw material, material supply part which supplies raw material to modification part, combustion part which burns the off gas of fuel gas from fuel cell and heats modification part, combustion-air supply part which supplies combustion air to combustion part, and modification temperature detection part.    USE - Fuel cell system for producing hydrogen-containing-gas.    ADVANTAGE - Appropriate excess air is maintained even when composition of source gas supplied to hydrogen generating apparatus changes.    DETAILED DESCRIPTION - A fuel cell system comprises electric-power-generated fuel cell using fuel gas containing hydrogen and oxidizing agent gas containing oxygen, hydrogen generating apparatus which has modification part for producing reformed gas which carries out steam reforming of raw material, material supply part which supplies raw material to modification part, combustion part which burns the off gas of fuel gas from fuel cell and heats modification part, combustion-air supply part which supplies combustion air to combustion part, modification temperature detection part which detects the temperature of modification part, electric power measurement part which measures the electric power generation amount in fuel cell, and control part which controls supply amount of combustion air to combustion part based on electric power generation amount in the fuel cell while controlling supply amount of raw material to modification part by material supply part so that detection temperature in modification temperature detection part which turns into preset predetermined preset temperature. An INDEPENDENT CLAIM is included for an operating method of fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of the process sequence in which control part determines supply amount of source gas to composition change of source gas and supply amount of combustion air to composition change of source gas. (Drawing includes non-English language text).    Acquiring temperature of modification part (S10)    Determination of supply amount of source gas (S11)    Acquiring electric power generation amount of fuel cell from electric power measurement part (S20)    Determination of supply amount of combustion air based on electric power generation amount (S21)