PURPOSE: To reduce a thickness and a weight by forming a protrusion similar to a sectional stepwise protrusion of a reinforcing rib inside the stepwise protrusion.

CONSTITUTION: A rib 25 protrudes by half punching a metal plate, and inside of the protruding plate is further made to protrude in multisteps. Accordingly, since a useless distortion does not occur at the time of forming the rib 25, the rib 25 can be accurately formed at a chassis 11 or a holder. Since the rib 25 can be provided in a wide range, a strength of the chassis 1, the holder can be improved. Since the rib 25 can be accurately and easily formed, deformations of the chassis 11 and the holder are reduced, and a recorder/ reproducer adapted for reducing its thickness and weight to be easily processed can be formed.