To facilitate mounting on a support arm beneath a rack such as luminaire by providing an engagement plate part to be engaged with a top groove via a clearance part for sliding an engagement edge part of the top groove of a groove member integrated with a fixture.

A groove member 10 integrated with a luminaire for illuminating a rack bottom is engaged while a reinforce plate part 6 slides an engagement edge part 11-1 of a top groove 11 thereof at a fixed engagement plate part 5. On the other hand, an engagement protrusion 7 is plugged in an engagement pore 9 of a rack support arm 8, a mounting bracket 1 is rotated along the side face of the support arm, and a table top 3 is so placed into contact with the top face of the support arm 8. A restriction side part 4 of an engagement side part 2 pinches both side faces of the support arm 8, and positioning is made certain. Therefore, the mounting bracket 1 hardly slip off from the support arm 8.