A fuel cell system comprising: a fuel cell that is supplied with an anode gas and a cathode gas to perform power generation; an evaporation unit that evaporates a reforming water to generate water vapor; a reforming unit that reforms a source gas using the water vapor generated in the evaporation unit to form anode gas; a water tank that collects the reforming water before being supplied to the evaporation unit; a water supply passage through which the water tank and the evaporation unit communicate with each other to supply the reforming water in the water tank to the evaporation unit; a water conveyance source that is placed in the water supply passage or the water tank, and is able to switch between a positive rotatable state of supplying the reforming water in the water tank to the evaporation unit and a reverse rotatable state of returning the reforming water of the water supply passage to the water tank; a water sensor that is provided upstream of the evaporation unit and downstream of the water conveyance source in the water supply passage to detect the existence of the water; and a control unit to which a detection signal of the water sensor is input to control the water conveyance source, wherein the control unit executes water supply abnormality determination processing of the water supply to the water sensor side, in any one of before starting a power generation operation of the fuel cell, after finishing the power generation operation of the fuel cell, and during stop of the power generation operation of the fuel cell, and wherein the water supply abnormality determination processing includes a returning operation that returns the reforming water of the water supply passage to the water tank by reversely rotating the water conveyance source to empty the reforming water of the water supply passage, a water supply operation that supplies the reforming water of the water tank to the water supply passage by positively rotating the water conveyance source in a state of emptying the reforming water of the water supply passage, and obtains a physical quantity concerning the output of the water conveyance source that is required until the water sensor detects the water, and a determination operation that determines whether or not the physical quantity in the water supply operation is within a prescribed range, determines that the water supply to the evaporation unit is normal when the physical quantity is within the prescribed range, and determines that the water supply to the evaporation unit is abnormal when the physical quantity in the water supply operation is out of the prescribed range. a fuel cell that is supplied with an anode gas and a cathode gas to perform power generation; an evaporation unit that evaporates a reforming water to generate water vapor; a reforming unit that reforms a source gas using the water vapor generated in the evaporation unit to form anode gas; a water tank that collects the reforming water before being supplied to the evaporation unit; a water supply passage through which the water tank and the evaporation unit communicate with each other to supply the reforming water in the water tank to the evaporation unit; a water conveyance source that is placed in the water supply passage or the water tank, and is able to switch between a positive rotatable state of supplying the reforming water in the water tank to the evaporation unit and a reverse rotatable state of returning the reforming water of the water supply passage to the water tank; a water sensor that is provided upstream of the evaporation unit and downstream of the water conveyance source in the water supply passage to detect the existence of the water; and a control unit to which a detection signal of the water sensor is input to control the water conveyance source, wherein the control unit executes water supply abnormality determination processing of the water supply to the water sensor side, in any one of before starting a power generation operation of the fuel cell, after finishing the power generation operation of the fuel cell, and during stop of the power generation operation of the fuel cell, and wherein the water supply abnormality determination processing includes a returning operation that returns the reforming water of the water supply passage to the water tank by reversely rotating the water conveyance source to empty the reforming water of the water supply passage, a water supply operation that supplies the reforming water of the water tank to the water supply passage by positively rotating the water conveyance source in a state of emptying the reforming water of the water supply passage, and obtains a physical quantity concerning the output of the water conveyance source that is required until the water sensor detects the water, and a determination operation that determines whether or not the physical quantity in the water supply operation is within a prescribed range, determines that the water supply to the evaporation unit is normal when the physical quantity is within the prescribed range, and determines that the water supply to the evaporation unit is abnormal when the physical quantity in the water supply operation is out of the prescribed range.