A method for operating a refrigerating cycle apparatus having a low-stage compressor and a high-stage compressor connected to each other in series, wherein a refrigerant switching valve is connected to a discharge side of the high-stage compressor, the refrigerant switching valve comprises a low-stage side outlet connected to a low-stage side evaporator and a high-stage side outlet connected to a high-stage side evaporator , the low-stage side evaporator is connected to a suction side of the low-stage compressor and the high-stage side evaporator is connected to a suction side of the high-stage compressor , the method being characterised by : determining whether or not an oil balancing is required between the low-stage compressor and the high-stage compressor ; and performing the oil balancing so as to transfer oil from the high-stage compressor containing more oil to the low-stage compressor containing less oil when it is determined to perform the oil balancing, wherein the performing of the oil balancing comprises: opening both the low-stage side outlet and the high-stage side outlet of the refrigerant switching valve for a preset time, with the low-stage compressor and the high-stage compressor turned off, so as to discharge the oil from the compressor containing more oil to the refrigerating cycle; and introducing oil discharged to the cycle into the compressor containing less oil, and wherein after performing the step of discharging the oil to the cycle, the low-stage compressor and the high-stage compressor are all driven for a preset time, with the low-stage side outlet of the refrigerant switching valve open and the high-stage side outlet thereof closed, so as to introduce the oil within the cycle into the low-stage compressor . determining whether or not an oil balancing is required between the low-stage compressor and the high-stage compressor ; and performing the oil balancing so as to transfer oil from the high-stage compressor containing more oil to the low-stage compressor containing less oil when it is determined to perform the oil balancing, wherein the performing of the oil balancing comprises: opening both the low-stage side outlet and the high-stage side outlet of the refrigerant switching valve for a preset time, with the low-stage compressor and the high-stage compressor turned off, so as to discharge the oil from the compressor containing more oil to the refrigerating cycle; and introducing oil discharged to the cycle into the compressor containing less oil, and opening both the low-stage side outlet and the high-stage side outlet of the refrigerant switching valve for a preset time, with the low-stage compressor and the high-stage compressor turned off, so as to discharge the oil from the compressor containing more oil to the refrigerating cycle; and introducing oil discharged to the cycle into the compressor containing less oil, and wherein after performing the step of discharging the oil to the cycle, the low-stage compressor and the high-stage compressor are all driven for a preset time, with the low-stage side outlet of the refrigerant switching valve open and the high-stage side outlet thereof closed, so as to introduce the oil within the cycle into the low-stage compressor . <EOS>