A ceiling-inserted fan includes a case having an air inlet on a bottom surface of the case and an air outlet on a side surface of the case, an adapter plate having a rectangular opening and forming at least part of the side surface of the case, a duct-connecting adapter protruding outward the adapter plate and an air-sending part for directing air from the air inlet to the air outlet. The duct-connecting adapter is separated from the adapter plate and includes a sealing section that contacts with a peripheral section of the rectangular opening inside the case and a locking section that contacts with the peripheral section of the rectangular opening outside the case. The sealing section and the locking section removably holds the peripheral section.