A gas fuel supply apparatus is constructed to be capable of supplying any one of propane gas and butane gas to a gas engine while switching between the propane gas and the butane gas. A check valve is provided in a gas fuel supply passages corresponding to a gas canister containing the gas fuel of a low working pressure (propane gas). The check valve prevents the gas fuel of a higher working pressure (butane gas) from flowing from a downstream side of the check valve to an upstream side of the check valve.