PURPOSE:To facilitate an installation of a grid onto a top plate and inform a user of an erroneous installation through a raising part of the top plate by a method wherein a projection at a lower part of a rear surface of the supporting grid is fitted to a recess formed in the top plate. CONSTITUTION:When a supporting grid is installed, a projection 9 of a supporting grid claw 8 is fitted to a recess 3 formed at a stepped pushing part 2, located at a position 4a, and another projection 9 of the supporting grid claw 8 is positioned at each of the positions 4b, 4c, 4d, and 4e contacted at an inner side with the stepped pushing part 2. At this time, in the case that a force of a rotational direction is applied to the supporting grid, this is restricted by the projection 9 of the supporting grid claw 8 location at a position 4a fitted to the recess 3. In addition, in the case that the forces of upward, downward, rightward or leftward directions are applied, the projection 9 of the supporting grid claw 8 is restricted at each of the positions 4a, 4b, 4c, 4d and 4e with the end surface of the stepped pushing part 2 as well as the recess 3, resulting in that the position setting of the supporting grid is carried out. In the case that the supporting grid is erroneously installed in a rotating direction, the projection 9 of the supporting grid claw 8 is placed on the rising part 5 arranged at the stepped pushing part, resulting in that the supporting grid is inclined to inform a user of an abnormal state in a quite easy manner.
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