Abstract: A ventilation fan for duct being capable of inserting the ventilation fan smoothly into the opening in the ceiling, and free from strange feel between the design of the decorative plate and pattern of the ceiling.It includes (a) a ventilation fan main body having a blower including a fan installed inside, an intake port formed at the lower side and a scroll having an exhaust port, and a wind tunnel installed at the intake port side of the blower, and having a curved portion rotatable along the brim of an opening formed in the ceiling, (b) an adapter for duct connection detachably fitted to the exhaust port, and (c) a decorative plate which may be installed by covering the lower surface of the wind tunnel.Therefore, as the curved portion of the wind tunnel rotates along the brim of the opening, the ventilation fan main body is fitted to the ceiling, and the decorative plate is fitted to the lower side by covering the wind tunnel. Moreover, the ventilation fan main body is connected to the adapter.