Structures are formed in a shape protruding from a wing surface, and a plurality of riblets are formed in a shape depressed from the wing surface. A first cross section obtained by cutting the structure by a flat face that is parallel to a flow and perpendicularly intersects with the wing surface has an inclined side that extends from a point on the wing surface to a top that is a point apart from the wing surface. An inter-structure flow channel is formed between two adjacent structures among the plurality of structures. The area of a face in one of the two structures and the area of a face in the other structure with which a fluid flowing in the inter-structure flow channel comes into contact are different from each other. Accordingly, peeling of the flow can be suppressed and the frictional resistance of the flow can be decreased.