Gas fuel supply apparatus

A gas fuel supply apparatus (12) is constructed to be capable of supplying any one of propane gas and butane gas to a gas engine (11) while switching between the propane gas and the butane gas. A check valve (42) is provided in a gas fuel supply passages (34) corresponding to a gas canister (13) containing the gas fuel of a low working pressure (propane gas). The check valve (42) prevents the gas fuel of a higher working pressure (butane gas) from flowing from a downstream side of the check valve (42) to an upstream side of the check valve.