The present invention relates to a cassette casing for a magnetic tape cassette, in which particularly when not in operation for recording and/or reproducing a signal, the cassette casing is tightly closed to protect a magnetic tape from being adhered with dusts and touching of fingers. The cassette casing is provided at its front portion with an openable and closable lid and at its lower surface with a slidable slider, whereby upon non-use, reel shaft insertion apertures, a front opening and a cut-out portion are closed by the front lid and the slider and this closed state is held by the cooperation of the front lid and the slider. Also, an elastic engaging piece member having at its tip end a protrusion is formed near the cut-out portion of the cassette casing and an engaging aperture which is engaged with this protrusion is formed through the slide, whereby the sliding movement of the slider is restricted by the engagement between the engaging piece member and the engaging aperture.