Abstract: A method for controlling an absorption system, comprising the step of performing slow open control for opening a control valve of a heat-source fluid feeding pipe connected to a regenerator at a predetermined speed and controlling the heat input at the time of start; wherein said control valve is quickly opened up to a predetermined opening degree at the time of start and thereafter, said control valve is opened at said predetermined speed is provided. There is no loss in the starting time by improving an operation delay of a heat-source fluid control valve at the start of operations and it is prevented that the heat-source fluid is excessively supplied to the absorption system itself which has still a low temperature to affect an equipment-side boiler even after the valve fully opens.