A sub sealing projection of an elastic sealing member for a fuel tank is entirely angled, and shaped so as to satisfy 20.degree. .ltoreq. .theta..sub.1 (angle defined by its upper surface and a cylindrical sealing portion).ltoreq. 35.degree., 50.degree. .ltoreq. .theta..sub.2(angle defined by its lower surface and the cylindrical sealing portion).ltoreq. 70.degree., 0.4.ltoreq. B (radial distance between a portion having a diameter corresponding to an inner peripheral surface of the cylindrical wall portion and a leading end of the sub sealing projection)/A (maximum projecting distance of the sub sealing projection).ltoreq. 0.55, and a leading end portion of the sub sealing projection is shaped so as to satisfy a condition, 0.1.ltoreq. R (curvature radius of cross-section thereof)/C(length of a base portion of the sub sealing projection).ltoreq. 0.2.