Air conditioner having frost preventing member

An air conditioner using a zeotropic mixture refrigerant formed of a high boiling-point refrigerant and a low boiling-point refrigerant is provided with a first temperature sensor (S1) for detecting a temperature at an inlet of an outdoor heat exchanger (4) when the outdoor heat exchanger (4) serves as an evaporator in heating cycle, a second temperature sensor (S2) for detecting an outside air temperature, and a controller (100) for controlling an opening degree of an expansion valve (5) on the basis of a temperature difference between the outside air temperature and the inlet temperature of said evaporator. For example, the opening degree of the expansion valve (5) is controlled so that the temperature difference between the outside air temperature and the inlet temperature is equal to a predetermined value.