PURPOSE: To obtain a mold at a low cost, by shaping cavity insert fitting grooves by grinding work of a base plate and fitting to said grooves the cavity insert which has the lateral grooves due to grinding of inside bottom faces and inner wall faces by electrospark machining on the mold for molding a resin seal type semiconductor device.

CONSTITUTION: On a mold for molding a resin seal type semiconductor device the cavity insert fitting grooves 16a, 16b are shaped by grinding machining of a base plate 11 and at the cavity insert 17 the lateral grooves 18 are shaped by grinding of at least the parts of inside bottom faces and inside wall faces by electrospartk machining. Further the concavities formed at the crossing parts of said cavity insert fitting grooves 16a, 16b and said lateral grooves 18 by fitting said cavity inserts 17 to said cavity insert fitting grooves 16a, 16b are used as the cavities for resin sealing the semiconductor elements attached to a lead frame. A mold can be obtained at a low manufacturing cost in such a way.