A method for warming up a fuel cell system comprising; a fuel cell which generates electric current due to an electrochemical reaction between hydrogen and oxygen, and which supplies the electric current to a load, an air supply pipe for supplying air to the fuel cell as an oxidant gas, an air exhaust pipe for discharging exhaust air from the fuel cell to an atmosphere, a compressor provided on either of air supply pipe and air exhaust pipe, which carries the air and a communication pipe which returns exhaust air to air supply pipe, and which communicates exhaust air pipe with air supply pipe to form a circulation cycle including compressor. The method of the present invention comprises: detecting a temperature of the fuel cell, and if the temperature of the fuel cell is lower than a first prescribed temperature, heating fuel cell by circulating the air, which has been heated by a heat generated due to adiabatic compression through compressor prior to supplying the electric current to the load from the fuel cell.