Recentry, much attention is paid to the co-generation system which consists of both a power generator, such as combustion gas engine, and an absorption refrigerator. In this system, it is necessary to clarify the dynamic behavior of the absorption refrigerator for disturbances, such as flow rate change and inlet temperature change of the ho)t water. Moreover, downsizing of the absorption refrigerator is now desired. However, the operation becomes more sensitive to disturbances as the size of the refrigerator decreases. Is this study, the authors establish the dynamical model of the absorption refrigerator driven by hot water. This model is to be used as the tool for the design of a control system and refrigerator with good controllability. For this purpose, the model should be as simple as possible. Lumped, parameter models are adapted for the elements where this approximation is adequate. The validity of this model is confirmed by comparing the calculation results with the experimental results.