PURPOSE: To prevent generation of flashes on a surface of an exposed insert by a method wherein the exposed portion of the insert is perfectly contacted with a rubber series soft body due to elasticity thereof upon tightening a metallic mold.

CONSTITUTION: In case a metallic insert, non-metallic insert or a combined metallic and non-metallic insert utilized for electronic parts is sealed by forming a resin, a rubber series soft body 11 is provided on a split surface of at least one mold of split molds 6, 7 in order to prevent a sealing resin 3 from flowing out toward an exposed portion ofthe insert 4 by urging it against the insert 4 upon closing of the metallic mold. Further, corner portions of the rubber series soft body 11 are chamfered and are provided with relieves 12 in order to prevent deformation and damage of the rubber series soft body 11 due to compression upon it is urged against the insert 4.