A fuel cell system comprises a fuel cell; a fuel cell controlling converter; an oxidizing gas supplier that is configured to supply an oxidizing gas to the fuel cell; and a controller that is configured to control a voltage and a current value of the fuel cell. In a first power generation state, the controller sets the voltage and the current value of the fuel cell according to a required output, based on a current-voltage characteristic of the fuel cell. In a second power generation state, the controller sets the voltage and the current value of the fuel cell according to the required output and a required amount of heat, to a voltage and a current value that provide a lower power generation efficiency than a power generation efficiency in the first power generation state. The controller reduces the required amount of heat in a process of changing over a power generation state from the second power generation state to the first power generation state. The controller reduces a decrement in the required amount of heat per unit time when the required output is equal to or higher than a reference value, compared with a decrement when the required output is lower than the reference value.