To increase the versatility of a window unit by forming an L-shaped curved piece that is curved to the indoor side at the lower end of a flashing member provided on a lower cross rail of a window frame, holding a tonguelike tight member against the curved piece, and suspending a shielding plate from the indoor end of the curved piece.

Tonguelike tight members provided on the outer peripheral surfaces of an upper cross rail and jambs of a window frame are brought into contact with the open side of a building frame, and a gasket 13 that abuts to a fastener member 38 is provided on the indoor side face of the window frame so that airtightness and watertightness are secured without sealing from the outside. Next, a flashing-part tonguelike tight material 8 is mounted on a curved piece 6 at the lower end of a flashing 5 mounted integrally with the window frame 108, a space S between the tight material 8 and the building frame is set to within about 10 millimeters, and a shielding plate 7 is suspended from the indoor end of the curved piece 6 for flashing. Thus the space between the flashing member of the window unit and the building frame can be made smaller for versatility irrespective of the shape of an opening in the building frame, and construction work can be performed easily in a short time from indoors.