A method for operating an air conditioner, the air conditioner comprising: a refrigerant circuit in which an outdoor unit and a plurality of indoor units are connected together through communication pipes (11, 12, 13, 14) and which is configured to be able to perform a refrigeration cycle in which cooling and heating operations are performed in parallel with each other, the communication pipes (11, 12, 13, 14) including a first communication pipe and a second communication pipe which has a larger inside diameter than the first communication pipe, and a switching mechanism configured to change the directions of refrigerants flowing through the first and second communication pipes (11, 12) depending on whether a heating dominant operation to be conducted between a full-heating load operation and a balanced heating and cooling load operation is being performed in a first load region which ranges from a full-heating load to a partial-cooling load or a second load region which ranges from the partial-cooling load to balanced heating and cooling loads, wherein the air conditioner is operated such that the switching mechanism changes the directions of refrigerants flowing through the first and second communication pipes (11, 12) depending on whether the heating dominant operation to be conducted between the full-heating load operation and the balanced heating and cooling load operation is being performed in the first load region which ranges from the full-heating load to the partial-cooling load or the second load region which ranges from the partial-cooling load to balanced heating and cooling loads, and such that, in the first load region, the switching mechanism allows a high-pressure refrigerant to flow from the outdoor unit to the indoor units through the second communication pipe, and allows a low-pressure refrigerant to flow from the indoor units to the outdoor unit through the first communication pipe, and such that, in the second load region, the switching mechanism allows the high-pressure refrigerant to flow from the outdoor unit to the indoor units through the first communication pipe, and allows the low-pressure refrigerant to flow from the indoor units to the outdoor unit through the second communication pipe. a refrigerant circuit in which an outdoor unit and a plurality of indoor units are connected together through communication pipes (11, 12, 13, 14) and which is configured to be able to perform a refrigeration cycle in which cooling and heating operations are performed in parallel with each other, the communication pipes (11, 12, 13, 14) including a first communication pipe and a second communication pipe which has a larger inside diameter than the first communication pipe, and a switching mechanism configured to change the directions of refrigerants flowing through the first and second communication pipes (11, 12) depending on whether a heating dominant operation to be conducted between a full-heating load operation and a balanced heating and cooling load operation is being performed in a first load region which ranges from a full-heating load to a partial-cooling load or a second load region which ranges from the partial-cooling load to balanced heating and cooling loads, wherein the air conditioner is operated such that the switching mechanism changes the directions of refrigerants flowing through the first and second communication pipes (11, 12) depending on whether the heating dominant operation to be conducted between the full-heating load operation and the balanced heating and cooling load operation is being performed in the first load region which ranges from the full-heating load to the partial-cooling load or the second load region which ranges from the partial-cooling load to balanced heating and cooling loads, and such that, in the first load region, the switching mechanism allows a high-pressure refrigerant to flow from the outdoor unit to the indoor units through the second communication pipe, and allows a low-pressure refrigerant to flow from the indoor units to the outdoor unit through the first communication pipe, and such that, in the second load region, the switching mechanism allows the high-pressure refrigerant to flow from the outdoor unit to the indoor units through the first communication pipe, and allows the low-pressure refrigerant to flow from the indoor units to the outdoor unit through the second communication pipe.