A fuel cell system mounted on a vehicle, includes: a fuel cell configured to cause electrochemical reaction between an anode gas and a cathode gas to proceed; and a fuel cell casing that is configured to place the fuel cell therein, wherein the fuel cell casing has a bottom face that is formed in an approximately rectangular shape, the fuel cell casing is positioned such that longitudinal sides of the approximately rectangular shape are parallel to a left-right direction of the vehicle, and the bottom face has a rib that is extended only in a front-back direction of the vehicle is provided on the bottom face.