A pair of side plate portions of a duct unit are formed to continue from an upstream plate portion to a downstream plate portion. A partition wall portion partitions a space between the pair of side plate portions into branch flow paths and the other area. A plurality of flow-out openings formed in the downstream plate portion are smaller, in area and width in a first direction, than flow-in openings formed in the upstream plate portion. The pair of side plate portions are formed to extend along edges of the flow-in openings and edges of the flow-out openings. The edges of each of the flow-in openings and the flow-out openings are opposite to each other in the first direction. An interval between the pair of side plate portions in the first direction becomes gradually narrower from upstream to downstream.