To save energy for water cooling for cooling hot exhaust gas discharged during forced air cooling a heat treating apparatus and relax the water hammer phenomenon when the cooling water is stopped from flowing.

The heat treating apparatus comprises a heat exchanger for cooling hot exhaust gas with water and a closing means for stopping the cooling water from flowing in this order from the upstream of a water passage. The stopping means has an air-operated valve controlled by air pressure to open and close the valve. When a close instruction is sent from a controller to the closing means, compressed air from an air feed source is shut off to hold the air-operated valve open, and a speed controller controls the air escaping rate of the air-operated valve so as to gradually close the valve with taking a predetermined time of 0.5 sec or more.