PURPOSE: To improve the utilization efficiency of thermal energy in the high- temperature treatment, e.g. retort sterilization, by carrying out the heating and cooling of the object to be treated successively with a definite time lag using plural high-temperature treatment tanks.

CONSTITUTION: When the heat treatment of the object in the first tank 10A is finished, the hot water W (≥120°C) in the tank 10A is transferred through the pipe 20 to the second tank 10B, and mixed with the cold water remaining in the second tank 10B. A part of the cold water in the tank 10B is heated with the heater 14 and sprayed in the tank 10B through the sprayers 12 to effect the heating of the object. In the above process, the hot water remaining in the tank 10A is cooled with the heat-exchanger 16, and the cold water thus obtained is sprayed through the sprayers 12 to cool the object. The procedures are repeated alternately using the above two tanks.