A fuel supply device for a gas engine is provided that includes a vaporizer, a primary regulator, a fuel changeover valve, a second inlet passage to which is introduced a second fuel fed out from a second fuel source, and a common outlet passage and can selectively connect the first and second inlet passages to the outlet passage, a common secondary regulator that regulates the pressure of the first fuel or second fuel so that it is at substantially atmospheric pressure and supplies it to a mixer, and a cutoff valve that opens/closes a fuel passage between the fuel changeover valve and the secondary regulator in response to operation/stopping of a gas engine, the vaporizer, the primary regulator, the fuel changeover valve, the cutoff valve, and the secondary regulator being integrally connected to each other to form a unit.