Abstract: A method for treating the interior surfaces of holes as well as the surfaces of a perforated workpiece by dipping it into treating liquid includes a step of causing treating liquid to flow at both ends of the holes in a direction perpendicular to these holes at different velocities. In order to create these liquid flows at different velocities, the workpiece is dipped in the liquid so that the holes extend substantially horizontally, and air bubble jets are alternately delivered along both ends of the holes, or fins are provided at locations adjacent both ends of the holes so as to extend outwardly upwardly at one side of the hole ends and extend outwardly downwardly at the other side of the hole ends, and these fins are moved upwardly and downwardly.