In construction of a plastic molded semiconductor package provided with a heat sink and electrically conductive leads attached to the periphery of the heat sink, a conventional electrically insulating, adhesive bonding tape is replaced by a non-adhesive insulating sheet and support bars are locally provided in a lead frame between the leads. For uniting the leads to the heat sink, the support bars are fixed to the heat sink whilst sandwiching the insulating sheet. Elimination of the adhesive bonding tape greatly reduces intrusion of contaminants into the package, simplifies the production process and lowers the production cost.