A cassette casing for a magnetic tape cassette has a casing body and a slider member mounted for sliding motion between a closed position and an open position. The slider member has a pair of guide grooves formed on the outer surface thereof remote from the casing body and extending from a first end of the cassette casing toward the other end, each of the guide grooves having first and second engaging holes which are formed on the bottom of the guide groove to extend through the slider member and open to the inner surface of the same and are spaced from each other in the longitudinal direction of the guide groove. The casing body is provided with a pair of locking projections each of which is resiliently urged toward one of the guide grooves from the inner surface of the slider member and is adapted to be selectively engaged with the first and second engaging holes so that its tip portion projects into the guide groove. The locking projections is disengaged from the engaging holes by a pair of lock release projections which are provided on a surface of a cassette deck and slide along the guide grooves when the magnetic tape cassette is loaded in the cassette deck. The outer surface of the slider member which is opposed to the surface of the cassette deck on which the lock release projections are provided is cut obliquely at each side edge portion thereof from a portion close to the guide groove on the outer side thereof to the side edge.
