PURPOSE: To plan more effective defrosting of an air side heat exchanger by controlling a solenoid valve provided on upper one of upper and lower air side heat exchangers defined by dividing an air side heat exchanger according to information from a pressure sensor while an air source heat pump water chiller boiler is being operated in a defrosting mode. CONSTITUTION: An air side heat exchanger 2 is divided into upper and lower portions and the upper portion is provided with a solenoid valve 9 and a check valve 10, and refrigerant flows through the solenoid valve 9 during defrosting cycle and flows through the check valve 10 during heating cycle. When a high-pressure pressure sensor 7 detects a first defrosting-completion judging value after defrosting operation is started, a controller 6 judges that frost on the air side heat exchanger 2 has been removed to a certain extent, frost still remains partially, portion where defrosting has been completed is of low thermal efficiency so that high-pressure pressure abruptly rises and the high pressure sensor 7 judges that defrosting-completion judging value has been detected. The controller 6 closes the solenoid valve 9 in order to stop allowing refrigerant gas to flow into upper side of the heat exchanger and make the refrigerant forcedly flow into lower side of the heat exchanger.
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