To easily and surely determine whether air is mixed in a cooling water circuit for cooling an engine or not without increasing the number of part items, in an air conditioner in which a compressor is driven by an engine.

The air conditioner 10 including the engine 40 for driving the compressor 15 constituting a refrigerant circuit 1, and the cooling water circuit 50 for circulating cooling water for cooling the engine 40 further has a circulation pump 57 for circulating the cooling water, and an air mixing determining section 60 for determining whether the air is mixed in the cooling water circuit 50 or not on the basis of a time to reach a predetermined set rotational frequency from a rotational frequency based on the actual rotational frequency of the circulation pump 57.