The magnetic tape cassette (20) wherein a closing lid unit (27) which covers a front side of the cassette main body (24) consists of three lid plates, an outer lid (29), a top lid (30) and an inner lid (31) has a structure wherein top ends of the inner lid guide grooves (35) open wide to an upper surface of the cassette main body (24) so that the inner lid slide pins (31c) of the inner lid (31) can be fitted in the inner lid guide grooves (35) from the upper side of the cassette main body (24). Slits (40a) and (40b) are provided at both ends of the top lid plate (30a)of the top lid (30) so as to facilitate flexibility of the outer lid linking portions (30b) and arm portions (30c) to be bent outward. Chamfered members are also provided to facilitate vertical insertion of top lid slide pins (230d) into top lid guide grooves.