Fuel cell system

A fuel cell system is provided which includes a fuel cell (1), an evaporation unit (2), a reforming unit (3), a water tank (4), a water supply passage (8), a water conveyance source (8A), a water sensor (87), and a control unit (100). The control unit (100) executes water supply abnormality determination processing of the water supply to the water sensor side. The water supply abnormality determination processing includes returning a reforming water of the water supply passage (8) to the water tank (4), supplying the reforming water of the water tank (4) to the water supply passage (8) in a state of emptying the reforming water of the water supply passage (8), obtaining a physical quantity concerning the output of the water conveyance source (8A) that is required until the water sensor (87) detects a water, and determining that the water supply to the evaporation unit (2) is abnormal when the physical quantity in the water supply operation is out of a prescribed range.