A tape cassette comprises a cassette case for accommodating a tape therein, an angularly rotatable lid provided at a front of the cassette case, where the lid is free to open and close, and a mechanism for maintaining the lid in an open state and a closed state, respectively. The mechanism comprises a slide rod making contact with an outer peripheral surface of the lid in the vicinity of a rotary fulcrum of the lid, a leaf spring for urging the slide rod towards the outer peripheral surface of the lid so as to make contact with the outer peripheral surface, a first groove provided in a side of the cassette case, into which the slide rod is slidably fitted, a second groove provided in a side of the cassette case for accommodating the spring member, into which a rear part of the slide rod can enter to permit sliding of the slide rod, and a rib portion integrally formed with the cassette case. The rib portion projects to cover a part of a side of the second groove at least at a position opposing a side of the slide rod fitted within the first groove, and the slide rod is prevented from escaping from the first groove by the rib portion.