A magnetic head inserting hole (6) is provided in the side face (4) of a flat cassette shell (2) comprising a top face (21), a bottom face (22), and side faces (23). A shutter member (8) is slidaly provided and consituted integrally of an upper sleeve section (81), a shutter section (82), and a lower sleeve section (83) which extend respectively along the top, side, and bottom faces (21, 4 and 22) of the shell (2) in such a way that the member (8) has a U-shaped cross section and opens and closes the hole (6). A guide claw (83a) protruded inward is provided to at least one of the upper and lower sleeve sections (81 and 83) of the member (8). A guide groove (22a) into which the claw (83a) is inserted is formed in the top or bottom face (21 or 22) of the shell (2) in the direction where the member (8) slides. A slit (83b) is provided in the corner between the upper or lower sleeve (81 or 83) and the shutter section (82) of the member (8) near the guide claw (83a) so as to improve the elasticity of the upper or lower sleeve section (81 or 83) when the section (81 or 83) is bent open. Therefore, when the shutter member (8) is attached to the shell (2), the guide claw (83a) is not in contact with the surface of the shell (2) and shavings of the shell (2) are not produced accordingly.