In a fuel cell system, an adhesion detection apparatus for detecting adhesion of a second switching valve for a sub tank is provided. The adhesion detection apparatus opens a first switching valve and a second switching valve for switching to a double switching valve open state where a fuel cell performs power generation, and thereafter, closes the first switching valve and opens the second switching valve for switching to a second switching valve open state where the fuel cell performs power generation, and detects adhesion of the second switching valve based on a change of a detection value of a pressure sensor in the second switching valve open state.