A fuel cell system includes: a fuel cell which performs electric power generation using a fuel gas containing hydrogen and an oxidant gas; a first supply pipe which supplies the fuel gas to the fuel cell; a first discharge pipe which discharges a fuel off gas and water led out from the fuel cell, by electric power generation of the fuel cell, to the outside; a second supply pipe which supplies the oxidant gas to the fuel cell; and an exhaust and drain valve which is disposed in the first discharge pipe, and regulates or allows the fuel off gas and the water to be discharged to the outside. The exhaust and drain valve includes a housing, a valve body, a diaphragm, a first port, a second port, a third port, and a valve seat.