PURPOSE: To raise the efficiency of construction by a method in which a reinforcing part of a waved cross section is provided for panels, and the first plate part and the second plate part are provided to the upside and downside of the reinforcing part in order to form a flat roof panel.

CONSTITUTION: A waved metal plate 12 as a reinforcing part securing strength is provided, the first slate board 16 is fixed to the upside of the wave top 14 of the metal plate 12, and the second slat board 19 is fixed by tapped screw 20 to the downside of the wave bottom 18. A cushion part 25 is inserted between the boards 16-1 and 16-2 of the flat roof panels 10-1 and 10-2 and fixed by tapped screw 20-3, and the upside of the panel 10 is covered with a waterproof layer 48. The weight of the structure can thus be reduced.