FUEL CELL SYSTEM

A fuel cell system includes a fuel cell that generates power using a fuel gas and an oxygen-containing gas, a reformer including a vaporizing section and a reforming section, a raw fuel supply that supplies raw fuel, a reformed water supply that supplies reformed water, and a controller. The controller has multiple calculation formulas for calculating an amount of reformed water to be used in the reforming section in response to a requested power level from an external unit, and selects, based on an increase or a decrease in a requested current level from the external unit, a formula from the multiple calculation formulas in response to the increase in the requested current level, and a formula different from the formula to be selected for the increase from the multiple calculation formulas in response to the decrease in the requested current level.