A molding die for molding a magnetic tape cartridge case in which an electronic part for exchanging a signal with the outside via contacts is incorporated as the need arises is such that a molding die portion for use in molding openings for exposing the contacts of the electronic part is constituted of two kinds of inserts which are alternatively attachable to the molding die proper; and a first insert is used to form openings for exposing the contacts, whereas a second insert is used to shut the openings for exposing the contacts.