A tape cassette including a cassette case consisting of an upper half and a lower half fastened together by at least one boss provided integrally on the upper half and at least one boss-receiving member provided integrally on said lower half. The boss includes an upper post and a plurality of first radial ribs projecting radially from the surface of the upper post and the boss-receiving member includes a lower post having a recess at its top end for receiving a projecting end of the boss, and the lower post is provided with a plurality of second radial ribs extending radially from the periphery of the lower post and said lower post has a minimum wall thickness around the recess sufficient to to connect the second radial ribs. The accuracy of the boss-receiving member is improved, the deviation of the total thickness of the upper and the lower halves which could otherwise occur is prevented, and the molding cycle can be shortened. The cassette further provides a lower half having a support rib of a reduced thickness for supporting a lock plate which is free of unsightly shrinkage mark due to imbalance of the thickness and yet has the same sufficient strength as the conventional thickness.