An air conditioner is disclosed. The air conditioner includes an indoor unit, a first outdoor unit including a first outdoor heat exchanger and a first compressor to be driven by electricity, a second outdoor unit including a second outdoor heat exchanger and a second compressor to be driven by an engine, a cooling water line, through which cooling water to cool the engine flows, and at least one waste heat-recovering heat exchanger for exchanging heat between refrigerant to be introduced into at least one of the first outdoor heat exchanger and the second outdoor heat exchanger and the cooling water flowing through the cooling water line. By provision of the waste heat-recovering heat exchanger, which heat-exchanges hot cooling water with refrigerant flowing through the first and second outdoor heat exchangers, frost formed on the first and second outdoor units during low-temperature heating is removed. It is possible to cope with a wide range of heating loads while achieving high efficiency, using both the electric heat pump (EHP) system and the gas engine-driven heat pump (GHP) system.