To provide an extrusion molding method of a resin enhancing the weatherability of a grained pattern and preventing discoloration or the like and a grained pattern resin coated steel pipe produced by this method.

Black, light brown or red-colored or multicolored heat-resistant pellets are mixed with a thermoplastic synthetic resin mixed with wood flour and good in softening, melting and flowing properties and the melt temp. of the colored heat-resistant pellets is set to 165°C or higher and the resulting mixture is melted in the heating cylinder, which is set to a heating temp. of 165-180°C over the range from the vicinity of the central part thereof to the front portion thereof, of an extrusion molding machine to be subjected to extrusion molding. A transparent resin having weatherability is applied to the surface of the molded grained pattern resin as a uniform thin film by extrusion molding.