A method includes: mounting a plurality of semiconductor elements on a substrate having wirings; connecting electrically electrodes of the semiconductor elements and the wirings; sealing the semiconductor elements with a resin, which is carried out by bringing a thermal conductor having a concavity and the substrate to be in contact with each other so that the semiconductor elements are positioned within the concavity and by filling the concavity with the resin; and separating respective semiconductor elements 1. In the resin-sealing step, in a state where the thermal conductor is arranged with its concavity facing up and the concavity of the thermal conductor is filled with a liquid resin, the semiconductor elements are dipped in the liquid resin in the concavity and the liquid resin is solidified. Due to these steps, a semiconductor device can be manufactured without experiencing troubles such as short circuit of the metal thin wires or imperfect filling of resin during the manufacturing steps, and thus semiconductor devices with stable quality can be manufactured.