An air conditioner comprising: a refrigerant circuit including at least one heat source unit having a compressor and a heat source side heat exchanger, a utilization unit (4, 5) having a utilization side expansion mechanism (41, 51) and a utilization side heat exchanger (42, 52), and a liquid refrigerant communication pipe and a gaseous refrigerant communication pipe for connecting the heat source unit to the utilization unit, the refrigerant circuit being capable of performing at least a cooling operation in which the heat source side heat exchanger functions as a condenser of the refrigerant compressed in the compressor and the utilization side heat exchanger functions as an evaporator of the refrigerant condensed in the heat source side heat exchanger; and a shutoff valve disposed at a position that is at once downstream of the heat source side heat exchanger and upstream of the liquid refrigerant communication pipe in a refrigerant flow direction in the refrigerant circuit in the cooling operation, and configured so as to be able to shut off the refrigerant flow; characterized in that the air conditioner further comprises: a liquid surface detection sensor provided at the heat-source side heat exchanger, configured to detect the amount of liquid refrigerant accumulated in the heat-source side heat exchanger, and disposed upstream of the shutoff valve in the refrigerant flow direction in the refrigerant circuit in the cooling operation, and configured to perform detection of the amount of liquid refrigerant located upstream of the shutoff valve by detecting a liquid surface height, wherein the liquid surface, is a boundary between the area where the refrigerant exists in a gaseous state and the area where the refrigerant exists in a liquid state, a memory configured to store, in advance, data on the required amount of refrigerant that is required for appropriately performing an air conditioning operation using the refrigerant circuit, and a control unit configured to perform the cooling operation with the shutoff valve closed based on a detection result of the liquid surface detection sensor and the required amount of refrigerant. a refrigerant circuit including at least one heat source unit having a compressor and a heat source side heat exchanger, a utilization unit (4, 5) having a utilization side expansion mechanism (41, 51) and a utilization side heat exchanger (42, 52), and a liquid refrigerant communication pipe and a gaseous refrigerant communication pipe for connecting the heat source unit to the utilization unit, the refrigerant circuit being capable of performing at least a cooling operation in which the heat source side heat exchanger functions as a condenser of the refrigerant compressed in the compressor and the utilization side heat exchanger functions as an evaporator of the refrigerant condensed in the heat source side heat exchanger; and a shutoff valve disposed at a position that is at once downstream of the heat source side heat exchanger and upstream of the liquid refrigerant communication pipe in a refrigerant flow direction in the refrigerant circuit in the cooling operation, and configured so as to be able to shut off the refrigerant flow; characterized in that the air conditioner further comprises: a liquid surface detection sensor provided at the heat-source side heat exchanger, configured to detect the amount of liquid refrigerant accumulated in the heat-source side heat exchanger, and disposed upstream of the shutoff valve in the refrigerant flow direction in the refrigerant circuit in the cooling operation, and configured to perform detection of the amount of liquid refrigerant located upstream of the shutoff valve by detecting a liquid surface height, wherein the liquid surface, is a boundary between the area where the refrigerant exists in a gaseous state and the area where the refrigerant exists in a liquid state, a memory configured to store, in advance, data on the required amount of refrigerant that is required for appropriately performing an air conditioning operation using the refrigerant circuit, and a control unit configured to perform the cooling operation with the shutoff valve closed based on a detection result of the liquid surface detection sensor and the required amount of refrigerant. a liquid surface detection sensor provided at the heat-source side heat exchanger, configured to detect the amount of liquid refrigerant accumulated in the heat-source side heat exchanger, and disposed upstream of the shutoff valve in the refrigerant flow direction in the refrigerant circuit in the cooling operation, and configured to perform detection of the amount of liquid refrigerant located upstream of the shutoff valve by detecting a liquid surface height, wherein the liquid surface, is a boundary between the area where the refrigerant exists in a gaseous state and the area where the refrigerant exists in a liquid state, a memory configured to store, in advance, data on the required amount of refrigerant that is required for appropriately performing an air conditioning operation using the refrigerant circuit, and a control unit configured to perform the cooling operation with the shutoff valve closed based on a detection result of the liquid surface detection sensor and the required amount of refrigerant.