To provide a refrigerant circuit and an operation checking method for improving reliability of a product by preventing leakage of a refrigerant, and also to provide a method for filling the refrigerant and a closing valve for filling the refrigerant enabling a supercritical refrigerant to be filled highly accurately and contributing to reduction of costs.

The refrigerant circuit is made into a pressure reduced state via a communication port 15 provided on a low pressure side between a pressure reducing unit 4 and an evaporator 5, and then the refrigerant is filled through the communication port 15 while a pressure sensor 19 on a high pressure side is being calibrated. The pressure reducing unit 4 is provided with a total-closure inhibiting function S.