To provide an air conditioner in which residual mineral oil in the existing piping for HCFC based refrigerant can be removed when it is reused and deterioration of performance due to intrusion of the residual mineral oil into the refrigeration or standing of oil on the inner surface of a heat exchanger can be prevented.

An oil separator 6 is provided at the joint of piping 9, 10 between a pressure reducing valve 4 and an indoor unit 8 such that first piping 13 and second piping 14 are inserted into a container from the bottom face thereof. The first piping 13 has one end coupled with the outdoor heat exchanger 3 side and the other end opening into the container whereas the second piping 14 has one end coupled with the indoor heat exchanger 8 side and the other end opening into a container 6. The first piping 13 has open end higher than the open end of the second piping 14 and a barrier wall is provided between both open ends. A gap is provided between the inner wall face at the ceiling of the container and the bottom face side such that a mixture of refrigerating machine oil flows mutually and an oil separator can be fixed and removed freely.