To easily attach a working table to a shelf, and to enlarge the opening area between left and right leg bodies.

Locking projections (stepped screws) 18 are projected inward at least in upper and lower parts of a lower inside surface of front side left and right columns 2 in a shelf 1. Connecting members 13 having a separate dimension between outside surfaces slightly smaller than a separate dimension between inside surfaces of the left and right columns 2 and having almost upward L-shaped engaging holes 16 capable of inserting the locking projections 18 are fixed to the rear ends of both leg plates 10 in a working table 9 by projecting backward to insert a rear end part of the working table 9 between the front side columns 2 of the shelf 1 to detachably engage the engaging holes 16 of the connecting members 13 with the respective locking projections 18.