To eliminate nonuniformity of cooling every portion of a metal tube causing deformation or surface irregularity in a heat treatment method for quickly cooling a heated metal tube using the spray nozzle row of a cooling liquid.

A cooling device 1 is disposed at the outer periphery of the metal tube 2 concentrically with respect to the metal tube 2, and is composed to cool the heated metal tube 2 by spraying a cooling liquid in a water film state to the outer surface of the metal tube 2. The slit inner edge 8 of an annular slit nozzle for spraying the cooling liquid to the metal tube 2 from an oblique direction at a definite angle is made to be projectable from an outer edge 9 toward the spray direction of the cooling liquid by a length L. Since the device is composed so that a liquid film collides on the same periphery as the perfectly circular external surface of the metal tube 2 at a prescribed angle, the circumference of the metal tube 2 is reliably, uniformly and quickly cooled.