An air conditioner capable of conducting a natural circulation operation by circulating refrigerant through an evaporator and a condenser located at a higher position than the evaporator, which are connected with pipes, wherein the air conditioner has means for obtaining an air conditioning load quantity to an outdoor air temperature in a temperature range, means for obtaining an air conditioning ability quantity to an outdoor air temperature in a temperature range in a case of using a predetermined mount of refrigerant, means for obtaining the maximum outdoor air temperature capable of conducting air conditioning at the time when an air conditioning load quantity produced from the means for obtaining an air conditioning load quantity substantially coincides with an air conditioning ability quantity from the means for determining an amount of refrigerant, as an amount to be charged, in which a maximum outdoor air temperature capable of conducting air conditioning among the obtained maximum outdoor air temperatures becomes the maximum.