A recording medium cassette comprises a cassette case cassette case board and a release part. The cassette case board has a plurality of board terminals and also has positioning holes formed in its outer surface so as to expose the plurality of board terminals along with insertion guide channels formed consecutively at the lower ends of the positioning holes so as to reach the lower end of the cassette case. Also, a release part slopes towards the lower end of the insertion guide channel away from the outer surface. The angle of the release part with respect to the height of the cassette case is greater than the angle of an inclined portion, which spans from contact points to free end parts of resilient connector contacts set up at a recording/playback apparatus connector, with respect to the height of the cassette case, and the connector contacts do not come into contact with the release part when the recording medium cassette is installed in the recording/playback apparatus and the contacts are therefore brought together in a reliable manner.