A case for a recording tape cartridge is formed by joining an upper case and a lower case via mutual peripheral walls. A blocking piece, which is disposed to extend along the peripheral wall of the upper case serving as a cover, is inserted into a recess, which is disposed in the lower case serving as a base and opens upward and sideward, and a portion of the recess is blocked off by the blocking piece, whereby an engagement hole is formed that is positioned lower than a parting line (PL) and opens only sideward. According to the invention, it is possible to form a recess that opens only toward the outer surface of the peripheral wall of the base of the case, without disposing undercuts in the base.