A fuel filler tube opening structure comprising: a fuel adaptor supported on a vehicle body of a vehicle and disposed inside the vehicle body; a fuel lid for openably closing an opening section formed in the vehicle body having the fuel adaptor supported thereon; and a protective cover provided on an inner surface of the fuel lid, wherein the protective cover surrounds an insertion opening portion of a mouthpiece, constituting a fuel filler tube opening, in spaced relation to the insertion opening portion to thereby prevent the protective cover from sticking to the insertion opening portion, and the protective cover has an eave section extending to a region above the fuel filler tube opening, characterized in that the fuel filler tube opening is oriented obliquely upward with respect to the vehicle; the eave section has a flange portion formed on an edge portion thereof and bent from the edge portion radially outward of the protective cover, the mouthpiece comprises a first mouthpiece, and a second mouthpiece, the second mouthpiece being fitted over one end portion of a feeder tube and being freely movably supported by a tongue portion 40a of an elastic member, which supports the fuel adapter on the vehicle body; the first mouthpiece has a lid ring section as a base section mounted to the distal end portion of the second mouthpiece, and a stepped section projecting from the inner end of the lid ring section toward the protective cover and then extending toward the center of the lid ring section, wherein the stepped section has the insertion opening portion; and the lid ring section has a fitting section formed on an outer peripheral end thereof, the fitting section fitting over a distal end portion of the second mouthpiece.. a fuel adaptor supported on a vehicle body of a vehicle and disposed inside the vehicle body; a fuel lid for openably closing an opening section formed in the vehicle body having the fuel adaptor supported thereon; and a protective cover provided on an inner surface of the fuel lid, wherein the protective cover surrounds an insertion opening portion of a mouthpiece, constituting a fuel filler tube opening, in spaced relation to the insertion opening portion to thereby prevent the protective cover from sticking to the insertion opening portion, and the protective cover has an eave section extending to a region above the fuel filler tube opening, characterized in that the fuel filler tube opening is oriented obliquely upward with respect to the vehicle; the eave section has a flange portion formed on an edge portion thereof and bent from the edge portion radially outward of the protective cover, the mouthpiece comprises a first mouthpiece, and a second mouthpiece, the second mouthpiece being fitted over one end portion of a feeder tube and being freely movably supported by a tongue portion 40a of an elastic member, which supports the fuel adapter on the vehicle body; the first mouthpiece has a lid ring section as a base section mounted to the distal end portion of the second mouthpiece, and a stepped section projecting from the inner end of the lid ring section toward the protective cover and then extending toward the center of the lid ring section, wherein the stepped section has the insertion opening portion; and the lid ring section has a fitting section formed on an outer peripheral end thereof, the fitting section fitting over a distal end portion of the second mouthpiece..