To provide an auxiliary top plate device capable of being simply fitted to or removed from a desk and setting its upper face on the same plane as a top plate by utilizing the drawer of the desk.

A pair of the right and left stoppers 12, 12 are protruded downward on the lower face of an auxiliary top plate having the lateral width size A larger than the size D between the right and left side plates 6, 6 of the drawer 2 of a desk 1 so that the distance E between the outside faces becomes nearly equal to the size D between the side plates 6, 6. The auxiliary top plate 11 is mounted on the upper face of the drawer 2 while the stoppers 12, 12 are kept in contact with the inside faces of both side plates 6, 6 of the drawer 2 drawn from the desk 1. The upper face of the auxiliary top plate 11 is made nearly flush with the upper face of the top plate 5 of the desk 1.