A fueling portion structure of a fuel tank, comprising: a fueling port member having a fueling port into which a fueling gun for fueling a fuel tank is inserted; an opening/closing valve that can open and close the fueling port; a rotation shaft for mounting, to the fueling port member, the opening/closing valve rotatably between a closed-valve position, at which the opening/closing valve closes the fueling port, and an open-valve position, at which the opening/closing valve is pushed by the fueling gun and opens the fueling port; and a spring member that is mounted to the fueling port member, contacts the opening/closing valve and urges the opening/closing valve toward the closed-valve position, the posture of the spring member changing such that a valve-closing direction component (F A1) of urging force, which the spring member applies to the opening/closing valve at the open-valve position, is smaller than a valve-closing direction component (F A0) of urging force that the spring member applies to the opening/closing valve at the closed-valve position, characterized in that the spring member comprises: a first anchor portion that is anchored on the fueling port member; a second anchor portion that is anchored on the opening/closing valve; and a bent portion that is formed by being bent between the first anchor portion and the second anchor portion, and whose position and bending angle can change in order to effect the change in posture, wherein the spring member is a torsion spring member. a fueling port member having a fueling port into which a fueling gun for fueling a fuel tank is inserted; an opening/closing valve that can open and close the fueling port; a rotation shaft for mounting, to the fueling port member, the opening/closing valve rotatably between a closed-valve position, at which the opening/closing valve closes the fueling port, and an open-valve position, at which the opening/closing valve is pushed by the fueling gun and opens the fueling port; and a spring member that is mounted to the fueling port member, contacts the opening/closing valve and urges the opening/closing valve toward the closed-valve position, the posture of the spring member changing such that a valve-closing direction component (F A1) of urging force, which the spring member applies to the opening/closing valve at the open-valve position, is smaller than a valve-closing direction component (F A0) of urging force that the spring member applies to the opening/closing valve at the closed-valve position, characterized in that the spring member comprises: a first anchor portion that is anchored on the fueling port member; a second anchor portion that is anchored on the opening/closing valve; and a bent portion that is formed by being bent between the first anchor portion and the second anchor portion, and whose position and bending angle can change in order to effect the change in posture, wherein the spring member is a torsion spring member. a first anchor portion that is anchored on the fueling port member; a second anchor portion that is anchored on the opening/closing valve; and a bent portion that is formed by being bent between the first anchor portion and the second anchor portion, and whose position and bending angle can change in order to effect the change in posture, wherein the spring member is a torsion spring member.