A digital amplifier integrated circuit (“IC”), which is a heat-generating electronic component, is mounted on an upper surface of an insulating substrate. The digital amplifier IC is disposed inside a recess of a heat sink. The heat sink includes a peripheral wall having an end face placed on the upper surface of the insulating substrate. A hot-melt resin layer, molded by hot-melt molding, covers the upper surface of the insulating substrate. The hot-melt resin layer is in contact with a side face of the peripheral wall and retains the heat sink on the insulating substrate. The recess of the heat sink has no hot-melt resin therein.