PURPOSE: To require no component for closing a hollow portion and improve the external appearance of an injection-molded hollow product by providing a thick-walled portion having the hollow portion in a resin semi-finished product at the time of injection molding, providing a cylindrical portion for injecting gas into the hollow portion on the semi-finished product, pressing a recessed portion provided at the top end of a heating head against the cylindrical portion and closing the cylindrical portion by melting it under heating.

CONSTITUTION: A hollow portion 35 is provided in a thick-walled portion 34 formed to a semi-finished product A, and a cylindrical portion 38 for gas injection is formed in a surface position of the thick-walled portion. Further, a heating head 43 has a recessed portion 45 at the top end thereof and the recessed portion is formed, for example, into a substantially semispherical recess. When the recessed portion 45 is pressed against the gas injection cylindrical portion 38, the top end of the gas injection cylindrical portion 38 is melted and deformed so as to be directed toward the center thereof along the inner face of the recessed portion 45, whereby an opening 38a at the top end of the gas injection cylindrical portion 38 is closed to form a closing 37. Since the heating head 43 allows the periphery of its recessed portion 45 to intrude into a recess 36, the position of the closing 37 becomes substantially the same level as a surface 21 of the semi-finished product A or lower than that.