To provide an auxiliary top board attaching structure for desk with which an auxiliary top board for placing a keyboard or the like can easily, strongly and attachably/detachably be fitted to the fore edge of top board at irreducibly minimum screwing positions.

In this attaching structure for attaching an auxiliary top board 5 to one part of fore edge of top board 1 for the desk, the fore part of connecting metal fixture 6 is attached to a lower surface 5A of auxiliary top board 5, a backward hook 7 is formed at a rear end part on both the sides of rear part of the connecting metal fixture 6 protruded backward rather than the rear edge of auxiliary top board, and a horizontal fixing board 9 having attaching holes 8 on both the side forward from these hooks is formed. Then, the backward hooks 7 are fitted and locked with lock holes 13, which are formed on a fore surface 12 of inner reinforcing member 11 laterablly attached to the depthwise intermediate part of lower surface of tope board, and both the fixing boards 9 are screwed on a lower surface 15 of fore reinforcing member 14, which is laterally formed at the fore edge of lower surface of top board 1, while using attaching holes 8.