PURPOSE: To shorten the cooling time to one by severals and to unnecessitate a long buffer interval before lubricating treatment and an intermediate stock part by cooling welded metal sheets with fluid, in particular, rust preventive oil.

CONSTITUTION: The welded plates or metal sheets 6 are mounted on a conveying unit 5 through a welder 3 and a smoothing brushing unit 2 and conveyed into a cooling unit 1. The fluid 9 is pumped up from a storage tank 8 by a pump 10 and forcibly fed into two supply conduits 12, 13 via a filter 11. Both supply conduits 12, 13 respectively emit the fluid 9 toward a welded seam of the welded metal sheets 6 guided into a bell-shaped spray chamber forming body 15, 16 by the conveying unit 5. The hot welded seam is rapidly cooled to less than 100°C where vaporization of the fluid is not generated anymore by the fluid which is washed out or sprayed for the welded seam heated at high temperature zone from both sides in the bell-shaped spray chamber forming body 15, 16.