PURPOSE: To improve the appearance eliminating the generation of sink or the like of a surface while reducing production cost of interior parts for automobile, by welding a sound absorbing or buffer material made of a synthetic resin on the internal surface of an interior material member molded with a thermoplastic resin.

CONSTITUTION: A conical projection 1b is premolded on the internal surface 1a of an interior material member 1 molded with a thermoplastic synthetic resin. A sound absorbing or buffer material 2 is abutted on the internal surface 1a of the interior material member 1 and pressed at the contact point with the projection 1b with an electrode 3 of a high frequency heater. The sound absorbing or buffer material 2 is melted at a weld part 4 and welded as melted inside from the top of the interior material member 1. In other words, the junction face between the interior material member 1 and the buffer material 2 is fastened by anchoring effect and both will not be peeled off.