PURPOSE: To make the boundary part where the colors are different from each other to natural appearance, by forming an outer layer of a skin material while a slide mold of a mold is projected inside from the mold main body under a heat insulating condition and then melt-adhering the second raw material on the inner-face of the outer layer of the skin material while the slide mold is returned to the mold face position and heat transfer is possible.

CONSTITUTION: A slide mold 3 is projected inside under the condition where it is heat-insulated from a mold main body 2 by means of its heat insulating part 3b and is sealed by using the first raw material box 6 where a raw material B1 is stored. Then, when a mold 1 is heated and an integrated body of the mold 1 and the first raw material box 6 is turned over by 180°, the raw material B1 flows down and reaches a mold face 2a to form an outer layer A. At this time, as the raw material B1 does not adhere on the slide mold 3, an opening part A' is formed. Then, the integrated body is returned to the original posture and the first raw material box 6 is exchanged with the second raw material box 7 where a raw material B2 is stored. Thereafter, the slide mold 3 is returned to an unprojected position, and the mold 2 is heated and the apex of the slide mold 3 is heated by means of an electric heater 5. The integrated body is turned over again and the same process is carried out to form an inside layer A2.