A hot water supply apparatus for combined use of an air conditioner comprising: an outdoor unit having a compressor in which first refrigerant is compressed and an outdoor heat exchanger in which the first refrigerant is heat-exchanged with outdoor air; a plurality of indoor units (4 and 6) connected to the outdoor unit and having an indoor heat exchanger in which the first refrigerant is heat-exchanged with indoor air; and a cascade hot water heater connected to the plurality of indoor units (4 and 6) in which the first refrigerant is heat-exchanged with second refrigerant, wherein the cascade hot water heater includes a capacity variable compressor which compresses the second refrigerant, a water refrigerant heat exchanger in which the second refrigerant compressed by the capacity variable compressor is condensed by heating water, an expander in which the second refrigerant condensed in the water refrigerant heat exchanger is expanded, and a cascade heat exchanger in which the second refrigerant expanded in the expander is heat-exchanged with the first refrigerant and then evaporated, wherein the cascade hot water heater is configured such that it modifies a flowing amount of the second refrigerant according to an outdoor temperature, an outlet water temperature of the water refrigerant heat exchanger and a working rate of the plurality of indoor units (4 and 6) and the cascade hot water heater, wherein the cascade hot water heater is configured such that it modifies the flowing amount of the second refrigerant by adjusting a maximum input frequency to be input to the capacity variable compressor, characterized in that the maximum input frequency is in proportion to a first reference value set according to a range of the outdoor temperature, a second reference value set according to a range of the outlet water temperature of the water refrigerant heat exchanger and a third reference value set according to the working rate of the plurality of indoor units (4 and 6) and the cascade hot water heater. an outdoor unit having a compressor in which first refrigerant is compressed and an outdoor heat exchanger in which the first refrigerant is heat-exchanged with outdoor air; a plurality of indoor units (4 and 6) connected to the outdoor unit and having an indoor heat exchanger in which the first refrigerant is heat-exchanged with indoor air; and a cascade hot water heater connected to the plurality of indoor units (4 and 6) in which the first refrigerant is heat-exchanged with second refrigerant, wherein the cascade hot water heater includes a capacity variable compressor which compresses the second refrigerant, a water refrigerant heat exchanger in which the second refrigerant compressed by the capacity variable compressor is condensed by heating water, an expander in which the second refrigerant condensed in the water refrigerant heat exchanger is expanded, and a cascade heat exchanger in which the second refrigerant expanded in the expander is heat-exchanged with the first refrigerant and then evaporated, wherein the cascade hot water heater is configured such that it modifies a flowing amount of the second refrigerant according to an outdoor temperature, an outlet water temperature of the water refrigerant heat exchanger and a working rate of the plurality of indoor units (4 and 6) and the cascade hot water heater, wherein the cascade hot water heater is configured such that it modifies the flowing amount of the second refrigerant by adjusting a maximum input frequency to be input to the capacity variable compressor, characterized in that the maximum input frequency is in proportion to a first reference value set according to a range of the outdoor temperature, a second reference value set according to a range of the outlet water temperature of the water refrigerant heat exchanger and a third reference value set according to the working rate of the plurality of indoor units (4 and 6) and the cascade hot water heater.