A semiconductor device that includes an insulating substrate having an upper conductor formed on an upper surface thereof and a lower conductor formed on a lower surface of the insulating substrate. The device also includes a semiconductor element mounted on the upper surface of the insulating substrate with an under-element solder therebetween. The device further includes a heat sink whereon the insulating substrate is mounted with an under-substrate solder therebetween. The device additionally includes a silicone gel covering the semiconductor element, the under-element solder, and the upper conductor. In addition, the device includes a filler covering the lower conductor and the under-substrate solder, without covering the semiconductor element, the under-element solder, and the upper conductor, and having a thermal conductivity larger than a thermal conductivity of air and a fluidity higher than a fluidity of the silicone gel.