A counter-rotating axial flow fan with reduced noise at the target operating point achieved without modifying a front impeller, a rear impeller, or a middle stationary portion is provided. An annular rib including a projecting surface for generating turbulent flow is formed on an inner wall portion of a casing at a position off from the middle stationary portion to a side of the rear impeller, the projecting surface extending radially inwardly of the inner wall portion and extending continuously in the circumferential direction of the inner wall portion. A fluid striking the projecting surface for generating turbulent flow is partially disturbed to form a turbulent flow before entering an area in which the rear impeller is provided. The turbulent flow suppresses flow separation of a fluid flowing along the surfaces of rear blades of the rear impeller from the surfaces of the rear blades.