A fuel tank comprising: a resin fuel tank body including an upper wall portion, a lower wall portion, and a side wall portion (lc) connecting the upper wall portion and the lower wall portion; and a reinforcing member (3A, 3B) integrally provided on an outer wall surface (12a, 12b) of the fuel tank body, the reinforcing member (3A, 3B) being a member which is inserted from outside the fuel tank body and attached thereto when manufacturing the fuel tank body, the reinforcing member (3A, 3B) reinforcing the outer wall surface (12a, 12b), characterized in that: the fuel tank further comprises a connecting member for partially connecting opposed inner wall surfaces (11a, 11b) of the fuel tank body, the connecting member having a cylindrical shape and including an upper end portion welded to the inner wall surface of the upper wall portion and a lower end portion welded to the inner wall surface of the lower wall portion; and/or an abutment portion where the opposed inner wall surfaces (11a, 11b) of the upper wall portion and the lower wall portion of the fuel tank body partially abut each other, wherein the abutment portion is formed by welding a first projecting portion (111, 131) projecting inwardly of the upper wall portion and a second projecting portion (112, 141) projecting inwardly of the lower wall portion together, and wherein a bottom portion (la2, la6) of the first projecting portion (111, 131) and a bottom portion (1b2, 1b6) of the second projecting portion (112, 141) abut each other, and in that the reinforcing member (3A, 3B) comprises: a base portion (31, 32, 33) disposed at a position corresponding to the connecting member and/or the abutment portion; and an extending portion (34, 35, 36, 37, 38) extending along the outer wall surface of the fuel tank body from the base portion (31, 32, 33), so as to suppress positive pressure deformation and negative pressure deformation of the fuel tank body. a resin fuel tank body including an upper wall portion, a lower wall portion, and a side wall portion (lc) connecting the upper wall portion and the lower wall portion; and a reinforcing member (3A, 3B) integrally provided on an outer wall surface (12a, 12b) of the fuel tank body, the reinforcing member (3A, 3B) being a member which is inserted from outside the fuel tank body and attached thereto when manufacturing the fuel tank body, the reinforcing member (3A, 3B) reinforcing the outer wall surface (12a, 12b), characterized in that: the fuel tank further comprises a connecting member for partially connecting opposed inner wall surfaces (11a, 11b) of the fuel tank body, the connecting member having a cylindrical shape and including an upper end portion welded to the inner wall surface of the upper wall portion and a lower end portion welded to the inner wall surface of the lower wall portion; and/or an abutment portion where the opposed inner wall surfaces (11a, 11b) of the upper wall portion and the lower wall portion of the fuel tank body partially abut each other, wherein the abutment portion is formed by welding a first projecting portion (111, 131) projecting inwardly of the upper wall portion and a second projecting portion (112, 141) projecting inwardly of the lower wall portion together, and wherein a bottom portion (la2, la6) of the first projecting portion (111, 131) and a bottom portion (1b2, 1b6) of the second projecting portion (112, 141) abut each other, and in that the reinforcing member (3A, 3B) comprises: a base portion (31, 32, 33) disposed at a position corresponding to the connecting member and/or the abutment portion; and an extending portion (34, 35, 36, 37, 38) extending along the outer wall surface of the fuel tank body from the base portion (31, 32, 33), so as to suppress positive pressure deformation and negative pressure deformation of the fuel tank body. the fuel tank further comprises a connecting member for partially connecting opposed inner wall surfaces (11a, 11b) of the fuel tank body, the connecting member having a cylindrical shape and including an upper end portion welded to the inner wall surface of the upper wall portion and a lower end portion welded to the inner wall surface of the lower wall portion; and/or an abutment portion where the opposed inner wall surfaces (11a, 11b) of the upper wall portion and the lower wall portion of the fuel tank body partially abut each other, wherein the abutment portion is formed by welding a first projecting portion (111, 131) projecting inwardly of the upper wall portion and a second projecting portion (112, 141) projecting inwardly of the lower wall portion together, and wherein a bottom portion (la2, la6) of the first projecting portion (111, 131) and a bottom portion (1b2, 1b6) of the second projecting portion (112, 141) abut each other, and in that the reinforcing member (3A, 3B) comprises: a base portion (31, 32, 33) disposed at a position corresponding to the connecting member and/or the abutment portion; and an extending portion (34, 35, 36, 37, 38) extending along the outer wall surface of the fuel tank body from the base portion (31, 32, 33), so as to suppress positive pressure deformation and negative pressure deformation of the fuel tank body. a base portion (31, 32, 33) disposed at a position corresponding to the connecting member and/or the abutment portion; and an extending portion (34, 35, 36, 37, 38) extending along the outer wall surface of the fuel tank body from the base portion (31, 32, 33), so as to suppress positive pressure deformation and negative pressure deformation of the fuel tank body.