PURPOSE: To produce a big cooling effect with a small amount of water by a method wherein an outer tube is provided with a pocket, opened so as to have an area enough to introduce the discharged amount of water from a spray nozzle with respect to a space, at the lower part thereof while the pocket is provided with a water discharging port at the lower part of the same.

CONSTITUTION: Cooling water, sprayed against an inner tube 28 in a slanted duct 4, flows down along the outer surface of the inner tube 28 or is collected by an outer tube 29 and is brought to the lower part of the same, then, enters a pocket 35 through a communicating port 36. In this case, cooling water in an upper part 6a does not flow down along the outer surface of the inner tube 52b whereby the amount of water, which flows down along the outer surface of the inner tube 52b, is comparatively small and the thickness of the water film thereof is comparatively thin. Accordingly, the cooling water, sprayed from the spray nozzle 69 in the lower part of the inner tube 52b, is collided surely against the outer surface of the inner tube 52b whereby a specified big cooling effect can be developed.