Semiconductor devices are produced by a process involving the steps of obtaining a core-box resin molded item inte­grated with a lead frame by coating said lead frame on its portions expected to be in non-contact with said core-box resin molded item with an organic high molecu­lar substance having a melting or softening point higher than the molding temperature of the resin constituting said core-box resin molded item and soluble in a solvent which does not dissolve said core-box resin molded item, placing the thus coated lead frame in position within a mold and injecting the resin into the mold thereby carrying out injection or transfe molding and immersing the lead frame bearing core-box resin molded item obtained in the foregoing step in said solvent to remove by dissolving said organic high molecular substance. Thus, resin flash formed on the lead frame can be removed with facility of the formation of flash can be suppressed without damaging the resin molded portion, whereby a good electrical connection between the lead frame and the semiconductor element can be accomplished. Further processes of the invention involve the use of ultrasound to help remove flash and also choice of materials for the lead frame and resin box with linear expansion coefficients which differ by less than a selected amount so as to achieve a good mutual adhesion and humidity resistance.