To enable a top plate vertically move without requiring a space around the top plate.

An auxiliary desk body 10 is equipped with a pair of mutually facing side boards 12. An elevator mechanism 30 is equipped on both sides of the top plate 26, vertically moves the top plate 26 almost in parallel with the upper board 14, and stops the top plate 26 at plural positions. A lock mechanism 32 is equipped near by on the elevator mechanism 30. The lock mechanism 32 can be operated to switch to a lock condition to obstruct the top plate 26 to move upward or downward and an unlock condition to allow the top plate 26 to move upward or downward. Only when the lock mechanism 32 is switched to the unlock condition, the top plate 26 can be moved upward or downward.