A magnetic tape cassette, particularly well adapted for high-density recording applications, in which the entry of dust and other contaminants is positively prevented. The cassette includes upper and lower half cases and a slide guard for closing an opening through which the tape can be extracted for recording and reproduction. The slide guard is in the form of a plate and is mounted so as to be slidable forwardly and backwardly of the cassette. A spring is provided to bias the slide guard towards the front end of the cassette. Protrusions are formed on the inner surface of the slide guard so as to be slidably in contact with the outer surface of the lower half case.