Cassette and changer apparatus for optical disk

A cassette comprising:

• an outer shell comprising an upper wall and a lower wall each comprising a square shape and a circular shape combined together, a side wall connecting between edges of the square shapes, and an opening portion provided between edges of the circular shapes;
• a plurality of trays, each comprising a concave portion in which an optical disk is placed, and an annular shape configured to support an outer peripheral edge of the optical disk, stacked on top of one another and stored between the upper wall and the lower wall;
• a shutter comprising at least a semicircular shape along the edges of the circular shapes of the upper wall and the lower wall, a gear on an outer peripheral wall of the shutter, and a plurality of shelves on an inner peripheral wall of the shutter each configured to support each of the trays along at least half a perimeter, the shutter configured to rotate along the edges of the circular shapes from a first position for closing the opening portion to a second position for opening the opening portion;
• and
• a lock mechanism between the side wall of the outer shell and a cylindrical surface along the outer peripheral wall of the shutter, and held by an elastic force in an engaging position in which the shutter is configured to be locked in the first position.