An air conditioning system includes a bypass passage and a cooling-heating switching mechanism. The bypass passage is configured to be at least a part of a flow passage connecting a third port to a refrigerant inlet of a compressor. A linear expansion valve opens and closes the bypass passage. The cooling-heating switching mechanism includes check valves and four-way valves. The four-way valve causes a refrigerant outlet of the check valve to communicate with one of the refrigerant inlet and a refrigerant outlet of the compressor. The four-way valve causes a refrigerant inlet of the check valve to communicate with one of the refrigerant inlet and the refrigerant outlet of the compressor and causes a port to communicate with the other of the refrigerant inlet and the refrigerant outlet.