Abstract: A magnetic tape cassette in which a mechanism for engaging a guard panel with a lower cassette half is simplified, the assembly process thereof is highly efficient, and the cassette is produced inexpensively and with excellent productivity. The magnetic tape cassette is constructed such that an engagement hole is formed on the inner surface of a side wall of a guard panel, and an engagement piece is formed on a side of the casette case and defined by a U-shaped slit so as to flex about a proximal end in the left and right directions relative to the cassette case. A locked state releasing projection and an engagement projection extend from the engagement piece. The engagement projection is engaged with the engagement hole when the guard is closed. The locked state releasing projection extends beyond the side wall of the guard panel and is used for releasing the engagement projection from the engagement hole.