To prevent the rain, snow, and water which have entered into the inside through a joint of siding from reaching a post and a sill.

A plate-like elastic resin material 20 secured to an outside surface of a furring strip 13 is provided on the rear side of a joint (25) of a siding 14. The elastic resin material 20 is equipped with, on its outside surface, a plurality of drain grooves 22 extending in longitudinal direction (claim 1). The elastic resin material 20 has water-proofing nature and has such restoration nature (swelling nature) as closes a hole part even if a nail is driven. So, the rain, snow, and water which have entered through a joint cannot farther intrude beyond the elastic resin material 20. Since the elastic resin material 20 is provided with a plurality of drain grooves 22, the rain, snow, and water which have entered through the joint fall along the drain groove 22 are discharged to the outside of a house.