FUEL CELL SYSTEM

A fuel cell system of the present invention includes: a reformer (1) configured to generate a hydrogen-containing fuel gas from a material gas and steam; a fuel cell (101) configured to generate electric power by using a fuel gas supplied from the reformer (1); a water tank (3) configured to store water; a water utilizing device configured to utilize the water supplied from the water tank (3); a first water supply unit (5) disposed on a water passage (30) extending from the water tank (3) to the water utilizing device and configured to supply the water in the water tank (3) to the water utilizing device; and a purifier (4) disposed on the water passage (30) and configured to purify the water flowing through the water passage (30), and the purifier (4) is provided such that when a water level of the water tank (3) is a full water level, the purifier (4) is filled with the water by the weight of the water.