A first rectangular pipe is connected along a path of an oxidation gas supply pipe that supplies an oxidation gas from a pump to an oxidant pole of a fuel cell. A second rectangular pipe is connected to an oxidation off-gas pipe connected to an outlet of the oxidant pole. A lower surface of the first rectangular pipe is contacted with an upper surface of the second rectangular pipe. A connecting path is provided to the contact surface, and a vapor permeable membrane is disposed at the connecting path. An upper surface of a third rectangular pipe is provided in surface contact with a lower surface of the second rectangular pipe, the third rectangular pipe being provided to a coolant out-pipe connected to a cooling jacket of the fuel cell. Generated water in the second pipe is heated and evaporated by a coolant heated in the third rectangular pipe. The water vapor then permeates the vapor permeable membrane, and is directed inside the first rectangular pipe so as to humidify the oxidation gas.