PURPOSE: To reduce the production cost and at the same time to realize the smooth inner surface finish of an intake manifold by a method wherein a core is fragmented and then removed by developing cracks in inorganic glass through locally quenching or locally rapid heating of the glass after the removal of the molded manifold from a mold.

CONSTITUTION: An inorganic glass core 1 for intake pipe, in which a nichrome wire 2 is inserted in zigzag form, is formed by molding a mold out of inorganic glass in the conventional insert molding technique. Next, an intake pipe is molded by employing the inorganic glass core 1 in the conventional molding means. After the molded pipe is removed from a mold and cooled down, electric current is allowed to pass through the nichrome wire 2 in order to heat the wire. Because the glass core has poor heat conduction, brittle cracks due to thermal strain develop at the position of the wire to initiate crack lines 3. Next, the fragmented glass core in the intake pipe 11 is easily removed by the action of water pressure so as to smoothly form the inner surface of the intake pipe.