To rearrange a shelf plate constituting a part of an upper shelf and to substantially expand and utilize a top plate area.
The upper shelf 12 is installed on the rear part upper surface side of a top plate 15. The upper shelf 12 is provided with shelf plates 24 and 25 and the shelf plate 25 is detachably provided. A groove 36 extended in a front-to-back direction is formed at two left and right parts on the lower surface side of the shelf plate 25, and at the time of installing the shelf plate 25 on a drawer 17, the upper end of the side wall part 39 of a drawer 17 and the groove 36 are engaged with each other. A keyboard 53 can be arranged on the shelf plate 25 rearranged in such a manner and a computer body and a display can be installed in a free area formed by detaching the shelf plate 25.