An air conditioner including a set of outdoor unit having a compressor that compresses a refrigerant, an outdoor heat exchanger, and a four-way valve; and a plurality of indoor units respectively having indoor heat exchangers connected to the outdoor unit through a liquid refrigerant pipe and a gas refrigerant pipe, in which the outdoor unit is arranged above the indoor units, comprises: on-off valves respectively provided in the indoor units on a side of the liquid refrigerant pipe; and a controller (71; 72; 73; 74; 75) that performs a defrost operation for circulating a refrigerant through the compressor, the outdoor heat exchanger, the liquid refrigerant pipe, the on-off valves, the indoor heat exchangers, and the gas refrigerant pipes in this order by switching the four-way valve, and the controller (71; 72; 73; 74; 75) once controls the on-off valves in a closing direction when the four-way valve switches the defrost operation to a heating operation that is performed for circulating the refrigerant through the compressor, the gas refrigerant pipe, the indoor heat exchangers, the on-off valves, the liquid refrigerant pipe, and the outdoor heat exchanger in this order, characterized in that the air conditioner further comprises: a first indoor-refrigerant pressure detector that detects a refrigerant pressure in the liquid refrigerant pipe in the indoor unit at the on-off valve on a side of the outdoor heat exchanger; and a second indoor-refrigerant pressure detector that detects a refrigerant pressure in the liquid refrigerant pipe in the indoor unit at the on-off valve on a side of the indoor heat exchanger; wherein the controller (71; 72; 73; 74; 75) controls the on-off valves when a refrigerant pressure that is input from the first indoor-refrigerant pressure detector becomes equal to or lower than a refrigerant pressure that is input from the second indoor-refrigerant pressure detector after the controller (71; 72; 73; 74; 75) controls the on-off valves in a closing direction when the defrost operation is switched to the heating operation. on-off valves respectively provided in the indoor units on a side of the liquid refrigerant pipe; and a controller (71; 72; 73; 74; 75) that performs a defrost operation for circulating a refrigerant through the compressor, the outdoor heat exchanger, the liquid refrigerant pipe, the on-off valves, the indoor heat exchangers, and the gas refrigerant pipes in this order by switching the four-way valve, and the controller (71; 72; 73; 74; 75) once controls the on-off valves in a closing direction when the four-way valve switches the defrost operation to a heating operation that is performed for circulating the refrigerant through the compressor, the gas refrigerant pipe, the indoor heat exchangers, the on-off valves, the liquid refrigerant pipe, and the outdoor heat exchanger in this order, characterized in that the air conditioner further comprises: a first indoor-refrigerant pressure detector that detects a refrigerant pressure in the liquid refrigerant pipe in the indoor unit at the on-off valve on a side of the outdoor heat exchanger; and a second indoor-refrigerant pressure detector that detects a refrigerant pressure in the liquid refrigerant pipe in the indoor unit at the on-off valve on a side of the indoor heat exchanger; wherein the controller (71; 72; 73; 74; 75) controls the on-off valves when a refrigerant pressure that is input from the first indoor-refrigerant pressure detector becomes equal to or lower than a refrigerant pressure that is input from the second indoor-refrigerant pressure detector after the controller (71; 72; 73; 74; 75) controls the on-off valves in a closing direction when the defrost operation is switched to the heating operation. a first indoor-refrigerant pressure detector that detects a refrigerant pressure in the liquid refrigerant pipe in the indoor unit at the on-off valve on a side of the outdoor heat exchanger; and a second indoor-refrigerant pressure detector that detects a refrigerant pressure in the liquid refrigerant pipe in the indoor unit at the on-off valve on a side of the indoor heat exchanger; wherein the controller (71; 72; 73; 74; 75) controls the on-off valves when a refrigerant pressure that is input from the first indoor-refrigerant pressure detector becomes equal to or lower than a refrigerant pressure that is input from the second indoor-refrigerant pressure detector after the controller (71; 72; 73; 74; 75) controls the on-off valves in a closing direction when the defrost operation is switched to the heating operation.