PURPOSE: To improve the weathering resistance of decorative surface, while said surface having expressive fine appearance of color is obtained by a method in which decorating agent is pressure-injected into a forming die simultaneously with extrusion-forming and is integrally pushed into the surface of the formed object.

CONSTITUTION: In a roof tile, when the inorganic material composed of cement or clay etc. is formed into the prescribed shape of tile section and is extruded from a nozzle 5 through a constricting die 3 and an expander die 4 as a base 2, the pigment 7 of preliminarily selected color is pressure-injected from the decorating agent-injecting pipe 6 provided at the part of the expander die 4, and is integrally buried in the extruded tile in the state where the pigment is exposed on the surface of the tile. When a plurality of the decorating agent- injecting pipe 6 are used and the pigments 7 of different colors are simultaneously injected, the decorative surface of multicolors may be easily formed.