The duct-type indoor unit of an air conditioner includes: a casing including a first surface and a second surface opposing each other, an inlet duct connection part which is formed in the first surface and to which an inlet duct is connected, and an outlet duct connection part which is formed in the second surface and to which an outlet duct is connected; a partition member partitioning interior of the casing into a first space on an inlet side and a second space on an outlet side, the partitioning member having an opening communicating the first space with the second space; a heat exchanger arranged inside the first space; and a centrifugal fan having an impeller with backward curved blades, the impeller being positioned inside the second space to suck in air in the first space through the opening. The impeller has a rotating shaft parallel to the first surface.