To improve operability by making an auxiliary top plate flush with a main top plate with a simple structure.

A sector notch section 3 is provided at one corner section of the main top plate 1 of a desk to form a part of the main top plate 1, and the first sector auxiliary top plate 10 kept flush with the main top plate 1 to form a part of the main top plate 1 and rotated in the horizontal plane is arranged at the notch section 3. The second sector auxiliary top plate 12 set flush with the first auxiliary top plate 10 is foldably connected to the first auxiliary top plate 10, and a rotary support shaft supporting the rotation of the first auxiliary top plate 10 is provided at the required portion of the notch section 3.