An apparatus for summing capacities of outdoor units in a multiple air conditioner comprises self-capacity storage devices mounted in outdoor units, respectively, for storing self-capacities of the outdoor units and outputting the self-capacities of the outdoor units when necessary, an outdoor unit capacity summing part mounted in one outdoor unit of the outdoor units for receiving the capacities of the outdoor units outputted from the respective self-capacity storage devices to calculate the total capacity of all the outdoor units, and a control device for receiving the result of the calculation performed by the outdoor unit capacity summing part to control the operation of the multiple air conditioner. The possibility of incorrect input of the capacities of the outdoor units is eliminated, and therefore, more accurate control of the multiple air conditioner is accomplished. Furthermore, it is not necessary for an engineer or a user to directly input the capacities of the outdoor units, and therefore, user convenience is improved.