PURPOSE: To relieve management of small parts such as a resin cap or the like with almost no effects on the appearance of a desk and moreover easily assemble or disassemble it by fitting the penetration edge and extended edge of a joint plate at the main top plate and the auxiliary top plate from the lowerface side by use of jointing bolts.

CONSTITUTION: This structure is provided with a side plate 1, a metal fixture 2 a first fixing bolt 31, a top plate 4, and a second fixing bolt 32. And at the side, a joint 5 and a slit 6 are formed. The auxiliary top plate 8 is made of a decoration plate 82 stuck to the six surfaces of the core material 81 forming a frame and has a curved shape as a fan between one side end and the other side end. And a recessed part 83 is provided at the underface 8a to be able to insert the extended side ends of respective jointing plates 71, 72, when the side end face 8b is made adjacent to the side end face 4b of the main top plate 4. The front end of the fixing bolt 33 inserted from the under face 7a side into the bolt insertion hole 7a of the jointing plates 71, 72 is fastened against a rasp-cut nut 84 hit in the recessed part 83. In this way, the auxiliary top plate 8 can be jointed with the main top plate 4 through the jointing plates 71, 72.