A fuel cell system includes a fuel cell body to generate electrical energy using a reaction of hydrogen and oxygen; a reformer to generate a reformed gas containing hydrogen by reforming fuel and to supply the reformed gas to the fuel cell body; a fuel tank to store the fuel in a partially liquefied state and to supply the fuel to the reformer; a case to encase the fuel cell body and the reformer; and a refrigeration unit attached to the case to store ambient air of the fuel tank, the ambient air of the fuel tank being cooled by latent heat of vaporization of the fuel.