A fuel-filling aperture opening and closing device comprising: a closing member (Ua) which is turned to an opening position by insertion of a fuel-filling nozzle (N) to open a fuel-filling aperture, wherein the closing member (Ua) comprises a pair of flap bodies and biasing means which position each of the flap bodies in a closing position by a biasing force, and the pair of flap bodies each comprises a turning assembly portion as a center of turning, and an abutment portion with respect to the other flap body , and is each configured to press a front surface portion thereof against a hole edge portion of a passage hole of the fuel-filling nozzle (N) by the biasing force and press the abutment portion against the abutment portion of the other flap body , the biasing means is a coil spring abutting at one spring end against a side of a support of the flap body , the fuel-filling aperture opening and closing device being characterized in that the coil spring has another spring end abutting against an inclined surface formed at a back surface portion of the flap body where a thickness gradually increases as it approaches the abutment portion , and each of the flap bodies is a plate-shaped body having a semicircular shape in plan view and provided with a semicircular arc edge portion along a circular arc corresponding to 180 degrees of a virtual circle having an outer diameter slightly larger than the hole diameter of the passage hole and a linear edge portion along a virtual segment dividing the virtual circle into two parts and passing through the center of the virtual circle, and the inclined surface is formed at a portion located on a virtual straight line (y) connecting a midpoint in a length direction of the semicircular arc edge portion and a midpoint in a length direction of the linear edge portion . a closing member (Ua) which is turned to an opening position by insertion of a fuel-filling nozzle (N) to open a fuel-filling aperture, wherein the closing member (Ua) comprises a pair of flap bodies and biasing means which position each of the flap bodies in a closing position by a biasing force, and the pair of flap bodies each comprises a turning assembly portion as a center of turning, and an abutment portion with respect to the other flap body , and is each configured to press a front surface portion thereof against a hole edge portion of a passage hole of the fuel-filling nozzle (N) by the biasing force and press the abutment portion against the abutment portion of the other flap body , the biasing means is a coil spring abutting at one spring end against a side of a support of the flap body , the fuel-filling aperture opening and closing device being characterized in that the coil spring has another spring end abutting against an inclined surface formed at a back surface portion of the flap body where a thickness gradually increases as it approaches the abutment portion , and each of the flap bodies is a plate-shaped body having a semicircular shape in plan view and provided with a semicircular arc edge portion along a circular arc corresponding to 180 degrees of a virtual circle having an outer diameter slightly larger than the hole diameter of the passage hole and a linear edge portion along a virtual segment dividing the virtual circle into two parts and passing through the center of the virtual circle, and the inclined surface is formed at a portion located on a virtual straight line (y) connecting a midpoint in a length direction of the semicircular arc edge portion and a midpoint in a length direction of the linear edge portion . <EOS>