PURPOSE: To enable a large number of ultra-thin type semiconductor devices with no defect to be manufactured at the same time by a method wherein a lead frame of prescribed shape is used, and the semiconductor devices are sealed up with resin by a specific molding die.

CONSTITUTION: A lead frame 10 is provided with a cutout 17 provided to a part of the side edge of one of side rails 15 located on one side of an aliment mount 11, a resin reservoir 19 formed adjacent to the side edge of the other side rail 15, a cutout formed inside the resin reservoir 19 as linked to it, and air reservoirs provided to both the side rails 15A. A molding die equipped with cavities 23, gates 21 provided as prescribed in size to inject sealing resin into the cavities 23, sub-runners 22 different in size depending on the location of the cavities 23 connected to them, and a runner 24 connected to the sub-runners 22 and tapered to be thin a its end is used to seal a semiconductor device of the design with resin.