To diminish simply and safely distortion and to improve a cooling rate and cooling capacity by alternately changing over the direction of the flow of cooling gas in opposite directions in a cooler body to be installed with a work and successively repeating the same.

A work 1 to be cooled after a heat treatment is installed in the cooler body 2 having a cap 11 and the cooling gas cooled with a heat exchanger 10 is supplied from the cooling gas supply piping 6 via valves 7a, 7b by a blower 5 and is cooled. The waste gas thereof is returned from cooling gas discharge piping 8 via valves 9a, 9b to the heat exchanger 10 and is circulated. At this time, the valves 7a, 9b are first opened and 7b, 9a are closed to pass the cooling gas from an upper gas opening 3 toward a lower gas opening 4. The valves 7a, 9b are closed and 7b, 9a are opened after the prescribed time to change over the cooling gaseous stream in the opposite directions. This changeover is repeated prescribed times by changing the blasting direction at every specified time.