NOVELTY - The fuel cell system comprises a reformer (2A) for forming an anode gas by reforming a source gas, a fuel cell (1), a source gas passage, a desulfurizer, a humidity and temperature sensor (510), a valve box, a flow volume sensor, and a control portion. The anode gas includes hydrogen. The fuel cell includes an anode (10) to which the anode gas is supplied and a cathode (11) to which a cathode gas is supplied. The source gas passage is configured for supplying the source gas to the reformer. The desulfurizer is provided at the source gas passage.    USE - Used as a fuel cell system.    ADVANTAGE - The cost-effective fuel cell system exhibits long life span and less power consumption.    DETAILED DESCRIPTION - The fuel cell system comprises a reformer (2A) for forming an anode gas by reforming a source gas, a fuel cell (1), a source gas passage, a desulfurizer, a humidity and temperature sensor (510), a valve box, a flow volume sensor, and a control portion. The anode gas includes hydrogen. The fuel cell includes an anode (10) to which the anode gas is supplied and a cathode (11) to which a cathode gas is supplied. The source gas passage is configured for supplying the source gas to the reformer. The desulfurizer is provided at the source gas passage for removing sulfur component included in the source gas. The humidity and temperature sensor is provided at the source gas passage for detecting a physical quantity in association with a temperature and a humidity of the source gas to be supplied to the desulfurizer, and is housed in a valve housing chamber of the valve box. The valve box forms the valve housing chamber covering a shut-off valve for closing the source gas passage. The flow volume sensor detects a flow volume of the source gas, which flows through the source gas passage. The control portion outputs information regarding a maintenance of the desulfurizer on a basis of the physical quantity in association with the temperature and the humidity of the source gas detected by the humidity and temperature sensor and a cumulative flow volume of the source gas flowing into the desulfurizer after a reference timing.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a fuel cell system.    Fuel cell (1)    Reformer (2A)    Anode (10)    Cathode (11)    Humidity and temperature sensor. (510)