Abstract: A temperature t.sub.hg of a solution 23a heated and concentrated in a generator 5, an outlet temperature t.sub.r of chilled water 21a flowed from an evaporator 1, and an outlet temperature t.sub.r of a flow of cooling water 22 fed from an absorber 2 to a condenser 4 are detected. A maximum allowable heating quantity Q* of generator 5 is calculated by means of a temperature t.sub.hg, t.sub.r and t.sub.c. A present heating quantity Q of generator 5 is controlled so as not to exceed a calculated maximum allowable heating quantity Q*. It is possible to prevent a heating quantity of the generator 5 from being excessively restricted and to prevent solution 23a from crystallizing at an outlet 6a of the heat exchanger 6. Further, loss of heat which occurs owing to returning generated refrigerant 24 to the concentrated solution 23a is obviated.