An air-conditioning device provided with a refrigerant circuit in which a compressor, a condenser, an expansion unit, and an evaporator are connected by piping, and in which flows a working fluid wherein a stabilizing agent has been added to a mixed refrigerant containing CF3I. The piping includes a rising gas pipe, which runs from the outlet side of the evaporator to the inlet side of the condenser via the compressor, and rises. The inner diameter of the rising gas pipe is no greater than an upper limit threshold value D1 obtained from formula (1).