A reactant gas channel part provided outside an auxiliary machinery case of a fuel cell system is connected to a case through hole. A plurality of channel seal members are provided between the auxiliary machinery case and the reactant gas channel part, where the channel seal members are arranged side by side in the radial direction of the case through hole and surround the case through hole. The reactant gas channel part has an opposing surface that faces a surrounding portion of the auxiliary machinery case that surrounds the case through hole. The channel seal members include an inner seal member abutting on the opposing surface and an outer seal member disposed radially outside of it. A connection channel communicating with an internal space of the auxiliary machinery case is formed between the inner seal member and the outer seal member.