To provide a method of manufacturing a plastic package wherein the spreading adhesion, burring, warpage or the like of copper of a plating pattern in a slit hole formation part can be prevented, and defective conductivity can be also prevented.

The method of manufacturing a plastic package 10 wherein a slit hole is to be formed includes a step to form a solder resist film which forms a conductor wiring circuit 17 and wherein a necessary part is exposed over the outside; a step to form a nickel and gold plating film layer in a part exposing over the outside from it; a step to make a through-hole 30 in a specific part wherein the drill of a router machining device for a wiring pattern 16 for electrolytic plating can pass through without being in contact with the hole 30, so that the hole may be brought into contact with the outer periphery of an individual body, by a drilling device with such a diameter that is larger than a pattern width and smaller than one not being in contact with the pattern; and a step to form a slit hole 24 by making the wiring pattern for electrolytic plating including the through hole 30 by the router machining device, so that a part for the outer circumference of the individual body may become an outer circumference.