All of hydrolysis resistance, heat resistance and calcium chloride resistance as the characteristics required during use are satisfied and the moldability as the characteristic required during molding is also ensured. Further, corrugation moldability, pressure proofness, bendability and shape retainability are ensured while enabling to adopt a mono-layer structure. A water pipe for use in automobiles molded with a thermoplastic polymer composition mainly comprising a polyamide resin (I) and a polyolefin (II), in which the polyamide resin (I) is a specified polyamide 9T; the amount of terminal amino groups of the polyamide resin (I) is 60 μmol/g or more and the amount of terminal carboxyl groups of the polyamide resin (I) is 10 μmol/g or less; the polyolefin (II) contains an acid modified polyolefin and the not acid-modified polyolefin at a mass ratio of from 80:20 to 20:80 and, as a result, the amount of the acid modification is from 0.2 to 0.5 mass %; and the mass ratio of (I) and (II) is from 90:10 to 70:30.