PURPOSE: To dissolve inconvenient treatment due to thermal collapse of tablets and to make automatic preparations for the tablets by a method wherein a preheating jig having holes enclosing surfaces other than in the inserting direction of thermosetting resin is provided using material that does not deteriorate in a high frequency electric field.

CONSTITUTION: Cylindrically formed tablets 1 are fitted in advance to a high frequency preheating jig 9 and set beteen the parallel-plate electrodes of a parallel-plate electrode type high frequency preheater to perform high frequency preheating. The tablets 1 are heated by high frequency until they take the prescribed preheating temperature, the jig 9 fitted with the tablets 1 is taken out of the high frequency preheater and set to the tablet inserting part of a mold along with the jig 9. Then, a grip 8 is pulled to open the shutter 7 and the heated tablets are dropped into the mold to resin-seal a semiconductor.