WE CLAIM:
1. A hollow molded article which has a joining site where two or more split objects are welded by plastic welding and which comes into contact with pressurized hydrogen, wherein the average spherulite size in a portion which is 500 um deep inside from the surface of said hollow molded article is 20 um or less, and the tensile strength of a test piece containing said joining site of said hollow molded article is 80% or more based on the tensile strength of a test piece not containing said joining site of said hollow molded article.
2. The hollow molded article according to claim 1, wherein the ratio of the average spherulite size in a portion which is 700 um deep inside from the surface of said hollow molded article to the average spherulite size in a portion which is 200 um deep inside from the surface of said hollow molded article is 1 or more and 2 or less.
3. The hollow molded article according to claim 1 or 2, wherein the standard deviation of the average spherulite size in a portion which is 500 um deep inside from the surface of each hollow molded article is 6 or less when said hollow molded article is divided into 16 pieces.
4. The hollow molded article according to any one of claims 1 to 3, wherein said hollow molded article is composed of a polyamide resin composition.
5. The hollow molded article according to claim 4, wherein the intensity ratio of a Raman band derived from crystals in the a form to a Raman band derived from crystals in the y form is 2.5 or less in a Raman analysis of a portion which is 100 um deep inside from the surface of said hollow molded article.
6. The hollow molded article according to claim 4 or 5, wherein said polyamide resin composition is a polyamide resin composition which is obtained by blending 0.01 to 10 parts by weight of an amide wax (B) with 100 parts by weight of

a polyamide 6 resin (A).
7. The hollow molded article according to any one of claims 4 to 6, wherein said polyamide resin composition is a polyamide resin composition obtained by blending a polyamide 6 resin (A) and a polyamide resin (C) which has a melting point of 245°C or less as measured by DSC and has a shorter rise time of the invariant Q than the rise time of the invariant Q of the polyamide 6 resin (A), said rise time of the invariant Q being measured by light scattering upon cooling from a temperature of 250°C at a rate of 20°C/min, and said polyamide resin (C) is blended in an amount of 0.01 to 5 parts by weight with respect to 100 parts by weight of said polyamide 6 resin (A).
8. A method of producing a hollow molded article which comes into contact with pressurized hydrogen, comprising a step of forming a hollow molded article by welding two or more split objects by a welding method selected from hot plate welding, infrared-ray welding, and infrared-ray/vibration welding in which vibration welding is carried out after a welded portion is warmed by infrared rays, wherein the average spherulite size in a portion which is 500 urn deep inside from the surface of said hollow molded article is 20 urn or less, and the tensile strength of a test piece containing a joining site of said hollow molded article is 80% or more based on the tensile strength of a test piece not containing a joining site of said hollow molded article.
9. The method of producing a hollow molded article according to claim 8, wherein said two or more split objects which constitute said hollow molded article are molded by a molding method selected from an injection molding method and an extrusion molding method.