<p>An impeller for a fan has multiple blades. On the surface of each blade are formed ridges or grooves having irregular shapes and extending substantially along the direction of the chord of the blade. Adjacent ridges of each blade preferably have different heights or different widths. Adjacent grooves of each blade preferably have different depths or different widths. Also, the width of each ridge or each groove preferably varies in the direction of the chord of the respective blade. The height of each ridge or the depth of each groove also preferably varies in the direction of the chord of the respective blade. In the impeller, separation of airflow on the blades is more effectively suppressed.</p>
