A piping connection structure comprising: a pipe body having an annular projection projecting radially outward at some distance from a fore end thereof; a resin tube into which a fore end side of the pipe body is to be inserted; a retainer formed in a hollow cylindrical shape, connected to an axial end of the resin tube, and restricting the pipe body from coming off by being engaged with the annular projection of the pipe body when the pipe body is inserted therein; and a seal member (71, 72) to be disposed in a radial gap between an inner circumferential surface of the resin tube and an outer circumferential surface of the pipe body; the resin tube comprising: a tube body portion; a first diameter-expanding portion formed to increase in diameter from an axial end of the tube body portion toward an open end of the resin tube, and axially positioning the seal member (71, 72) by holding the seal member in an axial gap with the retainer; and a second diameter-expanding portion formed to further increase in diameter from the first diameter-expanding portion, and to contact an outer circumferential side of the seal member and be connected to the retainer while in contact with the retainer; and the first diameter-expanding portion having the same or greater thickness than the tube body portion; the resin tube is formed by extrusion molding; the first diameter-expanding portion has a tapered portion; the first diameter-expanding portion comprises the tapered diameter-expanding portion on a side of the tube body portion, and a cylindrical portion on the open end side of the resin tube; the tapered diameter-expanding portion is configured to gradually increase in thickness toward the open end side of the resin tube; the tube body portion constitutes a main part of a fuel conveyor portion of the resin tube and has a first inner diameter D31; and the cylindrical portion has a second inner diameter D32 on a large diameter side being larger than the first inner diameter D31; the tapered diameter-expanding portion has the same diameter as the first inner diameter D31 on a small diameter side and the same diameter as the second inner diameter D32 on a large diameter side. a pipe body having an annular projection projecting radially outward at some distance from a fore end thereof; a resin tube into which a fore end side of the pipe body is to be inserted; a retainer formed in a hollow cylindrical shape, connected to an axial end of the resin tube, and restricting the pipe body from coming off by being engaged with the annular projection of the pipe body when the pipe body is inserted therein; and a seal member (71, 72) to be disposed in a radial gap between an inner circumferential surface of the resin tube and an outer circumferential surface of the pipe body; the resin tube comprising: a tube body portion; a first diameter-expanding portion formed to increase in diameter from an axial end of the tube body portion toward an open end of the resin tube, and axially positioning the seal member (71, 72) by holding the seal member in an axial gap with the retainer; and a second diameter-expanding portion formed to further increase in diameter from the first diameter-expanding portion, and to contact an outer circumferential side of the seal member and be connected to the retainer while in contact with the retainer; and a tube body portion; a first diameter-expanding portion formed to increase in diameter from an axial end of the tube body portion toward an open end of the resin tube, and axially positioning the seal member (71, 72) by holding the seal member in an axial gap with the retainer; and a second diameter-expanding portion formed to further increase in diameter from the first diameter-expanding portion, and to contact an outer circumferential side of the seal member and be connected to the retainer while in contact with the retainer; and the first diameter-expanding portion having the same or greater thickness than the tube body portion; the resin tube is formed by extrusion molding; the first diameter-expanding portion has a tapered portion; the first diameter-expanding portion comprises the tapered diameter-expanding portion on a side of the tube body portion, and a cylindrical portion on the open end side of the resin tube; the tapered diameter-expanding portion is configured to gradually increase in thickness toward the open end side of the resin tube; the tube body portion constitutes a main part of a fuel conveyor portion of the resin tube and has a first inner diameter D31; and the cylindrical portion has a second inner diameter D32 on a large diameter side being larger than the first inner diameter D31; the tapered diameter-expanding portion has the same diameter as the first inner diameter D31 on a small diameter side and the same diameter as the second inner diameter D32 on a large diameter side.