PURPOSE: To lighten the weight of external wall panels and prevent corrosion from being generated due to rainwater or the like by lining the whole external wall of a structure with external wall panels, to arrange window frames at window opening-sections, and by arranging continuous drain passages inside the external wall and inside the window frames.

CONSTITUTION: A frame body surface consisting of light gauge steels on external wall panels 1 is lined with fire-proof and water-proof panels 3 coated with fire-proof and water-proof material, and finish panels 4... to be composed with fixed spaces between them, and the spaces are formed as the drain passages 6 of infiltration rain and condensation. Besides, space sections between a window frame 7 and the external wall panels 1, 1 are filled up with joints 11, 12. Then, the head 8, sill 9, and jambs of the window frame 7 are formed in a rectangular hollow cross-sectional area, and a drain passage 13 is set. Besides, outside a room with the the sill 9 of the window frame 7, a drain port 15 communicated with the drain passage 15 is formed. As a result, rainwater and condensation water infiltrated inside the finish panels 4 can be drained from the drain passages 6, 13 and the drain port 5.