In a vehicle to which a wheel well rear portion structure has been applied, a curved portion of an upper wall of a guide portion bulges obliquely toward the vehicle lower side as seen in a side view and is connected to a discharge portion. An air flow flowing in a flow channel flows along the upper wall due to the Coanda effect, so in the air flow along the curved portion the flow component heading toward the vehicle rear side becomes larger. The flow direction of the air flow discharged from the discharge portion substantially coincides with the flow direction of an air flow flowing in the space under the body of the vehicle. In a state in which a drop in the flow speed of the air flow discharged from the discharge portion is controlled, the air flow can be caused to smoothly merge with the air flow.