An air-intake duct of the present invention, configured to guide air to a throttle device coupled to an engine, includes a main wall forming a main passage; and a sub-wall provided outside the main wall to form a sub-passage; wherein the sub-wall has a height smaller than a height of the main wall; and wherein the sub-passage is defined by an inner wall surface of the sub-wall and an outer wall surface of the main wall and is disposed on one side in a direction substantially perpendicular to a passage axis of the main passage.