To provide an air conditioner in which a cleaning operation time of transition piping can be shortened as further as possible, the air conditioner being installed by diverting existent transition piping.

An air conditioner 1 comprises an outdoor unit 2 and at least one indoor unit 3 connected to the outdoor unit 2 via liquid coolant piping 5 and gas coolant piping 4 and for the air conditioner 1, existent transition piping can be diverted as the liquid coolant piping 5 and the gas coolant piping 4. The air conditioner 1 also comprises a piping cleaning control section 25 for cleaning the existent transition piping through intermittent liquid back by opening/closing an electronic expansion valve 11 for cooling cyclically multiple times between an aperture A for overheat control and an aperture B that is nearly full opening or full closing, in a cooling cycle, during the piping cleaning operation after connecting the outdoor unit 2 and the indoor unit 3.