PURPOSE: To make it possible to shorten the start-up time and achieve good stability in controlling the start-up by a method wherein the flow rate of the cooling water is reduced to the lowest allowable value at the start and then increased corresponding to the differences between the temperature of the cooling water at the outlet and the cooling capacity and, when the temperature reaches a specified point, the flow rate of the cooling water is shifted to the rated value for the subsequent operation.

CONSTITUTION: An absorption-refrigerating machine 1 is equipped with temperature detectors 4, 5 for detecting the temperatures of the cooling water respectively at the inlet and at the outlet, with a flow rate detector 6 for detecting the flow rate of the cooling water from the absorption-refrigerating machine 1, and with a calculator 7 which computes the cooling capacity of the cooling water from the results of the detection by said detectors 4, 5, 6 and which finds, from the thus determined cooling capacity and the temperature of the cooling water at the outlet, the lowest limit of the flow rate of the cooling water. This absorption- refrigerating machine 1 has a flow rate regulator 8 and a flow rate control valve 9 as means of controlling the flow rate of the cooling water, and, when its operation is started, the flow rate of the cooling water is decreased to the lowest limit and, as the temperature of the cooling water at the outlet approaches a specified point, the flow rate of the cooling water is so controlled as to be increased on the basis of the differences between the temperature of the cooling water at the outlet and the cooling capacity; when the temperature of the cooling water has reached a specified point, the control shifts the flow rate of the cooling water to the rated value for the subsequent operation.