Fuel cell system and method of operating the same

A fuel cell system comprising:

• a reformer configured to generate a hydrogen-containing gas through a reforming reaction by using a raw material and steam;
• a raw material supply device configured to supply the raw material to the reformer;
• a steam supply device configured to supply the steam to the reformer;
• a temperature detector configured to detect a temperature of the reformer;
• a fuel cell configured to generate electric power by using the hydrogen-containing gas;
• a combustor configured to combust the hydrogen-containing gas discharged from the fuel cell to heat the reformer;
• and
• a controller configured to, while controlling the raw material supply device such that the temperature detected by the temperature detector becomes a target temperature, control the steam supply device in accordance with a change in a composition of the raw material, such that a change rate of a steam supply amount to the reformer becomes less than a change rate of a raw material supply amount to the reformer.