A fuel cell system is provided which performs simple control of the time until warm-up of the reformer (1) is completed. This fuel cell system comprises a reformer (1) which generates a reformate gas, and a fuel cell (2) which generates power from an oxidizing agent and the reformate gas supplied from the reformer (1). The fuel cell system is provided with a temperature sensor (7) which performs a detection at one position of the temperature immediately before starting or re-starting the reformer (1) and a controller (6) estimating the time to warm-up the reformer (1) based on the detected temperature of the reformer (1).