FUEL CELL SYSTEM, METHOD OF CONTROLLING FUEL CELL SYSTEM, AND FUEL CELL SYSTEM-MOUNTED VEHICLE

A fuel cell system FCS includes a fuel cell a high voltage circuit for driving an electromotor and a relay for electrically connecting or blocking the fuel cell to or from the high voltage circuit A control unit obtains insulation decrease information in accordance with a request for starting the fuel cell, and performs, when a specified insulation decrease occurred region is not a fuel cell region SE including the fuel cell and the cooling circuit, conductivity reduction process on cooling liquid using a conductivity reduction unit before having a relay connect, and has the relay connect after completing the conductivity reduction process.