To easily detach only a rack unit from the table top of a desk while attaching an illuminator on the table top of the desk as it is and to enable natural illumination in the case of arranging the desk at the window.

A lower fixed back board 9 and an intermediate fixed back board 19 are provided between upper and lower rack boards 11 and 16, and an intermediate space 28 sized enough for inserting/ejecting an illumination head 22 is secured between the lower fixed back board 9 and the intermediate fixed back board 19. A strut 20 of an illuminator 3 is arranged on the rear side of the lower fixed back board 9, and an arm 21 of the illuminator 3 is protruded forward from the intermediate space 28. The intermediate space 28 is covered with a freely attachable and detachable type back board 29.