To secure waterproof function between a ventilation louver and the external wall and prevent rainwater from accumulating between the roof space of an eaves soffit and the upper face of a ventilation louver, by arranging the upper face of the upper frame of a ventilation louver to contact the eaves Soffit through a waterproof material and fitting the rear side end face of the side frame and the lower frame to the external wall of the periphery of an opening through the waterproof material.

An opening 1a connected to a roof space is formed at the lower side of the ridge of an external wall 1 of a building. A framework 11 is formed by a chevron-shaped upper frame, side frames at both sides, and a lower frame. A ventilation louver 10 in which ventilating louvers 12 are parallelly and nearly horizontally arranged in the inside of the framework 11, is fitted to the external wall face of the outdoor side of the opening 1a so that the chevron-shaped upper face contacts the eaves soffit 2 through a waterproof material. And the rear face of the side frame and the lower frame is fiitted to contact the peripheral edge of the opening 1a of the external wall 1 through the waterproof material. The flexible waterproof material having a nearly square sectional shape is fitted along a continuous groove formed at the rear side of the upper face of the upper frame and the rear side of the side frame and the lower frame of the framework 11 in a half-embedded condition and also fitted endlessly at the peripheral edge of the ventilation louver 10.