PURPOSE: To provide a semiconductor device and a manufacturing method thereof where the device is remarkably enhanced in heat dissipating property and formed thin free from the movement of a semiconductor device, the occurrence of air bubbles, and resin non-filled sections at the package of the semiconductor device.

CONSTITUTION: A lead frame 1 mounted with a semiconductor device 10 is placed on a heat dissipating board or a synthetic resin board 20 provided with resin introducing holes 21, and the lead frame 1 mounted with the semiconductor device 10 is packaged together with the heat dissipating board or the synthetic resin board 20 into one piece. At this point, the lead frame 1 mounted with the semiconductor device 10 is placed on the heat dissipating board or the synthetic resin board 20, and the heat dissipating board or the synthetic resin board 20 is pinched between a cope 31 provided with a recess 32 where the semiconductor device 10 and the like are housed and a drag 35 provided with resin inlets 37 coincident with the resin introducing holes 21 provided to the heat dissipating board or the synthetic resin board 20, and resin is injected into the recess 32 provided to the cope 31 through the resin inlets 37 through the intermediary of the resin introducing holes 21.