PURPOSE: To facilitate the mounting of sidings.

CONSTITUTION: Right and left vertical frame materials 11 and 12 on which a siding plate material 3 is installed are formed as furring strips 11a and 12a of which mounting surfaces are projected by specified amounts from upper and lower lateral frame materials 13 and 14. Also a water-resistant sheet 2 is stuck between a square frame body 1 and the siding plate material 3. Thus, because a vent layer interconnected in vertical direction is formed between the siding plate material 3 and the water-resistant sheet 2, an external wall panel with water resistant and vent functions can be built easily.