PURPOSE: To shorten a freezer stop time due to crystallization and to enable rapid elimination of local corrosion environment by providing a control device which decides the occurrence of crystallization of a solution by means of a temperature detecting means and a pressure detecting means to output a decrystallizing command and controls decrystallizing operation.

CONSTITUTION: In a dense solution piping 11 of a solution return system from a high temperature reproducer 5, since a water - lithium bromide solution is high in concentration and cooled by radiation, it is apt to crystallize. The temperature of this solution return system is perceived by a temperature switch 12 and the pressure of the high temperature reproducer 5 by a pressure switch 13 to decide the occurrence of crystallization. When it is decided that crystallization occurs, a control device outputs a decrystallization command to a decrystallizing device, and circulation of cold water through a vaporizer 2 and circulation of cooling water through an absorber 1 and a condenser 4 are stopped. Heat is inputted to the high temperature reproducer by a heat input control device 15 as a solution pump 8 and a refrigerant pump 9 are run, and temperature and a pressure in a freezer are gradually increased. In this case, a solution feed valve 14 of a dilute solution piping 10 of a solution feed system is fully opened, and temperature and a pressure in the freezer are increased to a value higher than a crystallization freezing point. This method increases the degree of dissolution of an ambient solution crystallization of which is deposited to melt crystallization.