A cassette holder 2, with a locking hook 2a and a fish hook 2c beneath a lower face of the holder 2 and having a loaded cassette 1 therein, is pressed downward by pushing a top plate 4 and moved toward a chassis frame 6. A mis-insertion-proof panel 11 disposed on a back plate 7 by a supporting shaft is rotated on a rotation supporting shaft 14 by sliding along the lower face of the holder 2. When the cassette holder 2 reaches to a lower end position, the panel swings into a space between a rear face of the cassette 1 and the back plate 7, at the same time, the locking hook 2a is engaged with the sliding lock shaft (not shown), thereby completing a cassette loading. Contrary to the downward movement, in an upward movement of the holder 2 until its upper end position, the mis-insertion-proof panel 11 rotates on the rotation supporting shaft 14, and engaging shafts 12 provided on an edge of the panel 11 slides along the lower face of the holder 2 until the shafts 12 are engaged with the hook 2c. The hook terminal receiving the shaft 12 has a boss standing at a right angle with a locus of the shaft 12, which shapes the hook 2c into a "J" letter, the panel 11 thus prevents the cassette 1 from being inserted into the mechanism by mistake.