An air-conditioning apparatus (100, 200) capable of simultaneously performing a heating operation and a defrosting operation, the apparatus comprising: a main circuit including a compressor, a load side heat exchanger, a load side expansion device, a plurality of heat source side heat exchangers connected in parallel one another, and an accumulator that are connected by refrigerant pipes to form at least a heating circuit; a first gas bypass pipe branching off from the refrigerant pipe on a discharge side of the compressor and allowing refrigerant to flow into a defrosting-target heat source side heat exchanger of the heat source side heat exchangers; a plurality of first opening and closing devices arranged in the first gas bypass pipe, the first opening and closing devices each configured to permit or stop a flow of the refrigerant through the first gas bypass pipe; characterized by a second gas bypass pipe branching off from the refrigerant pipe on the discharge side of the compressor and allowing the refrigerant to flow into the accumulator; at least one second opening and closing device disposed in the second gas bypass pipe, the second opening and closing device configured to permit or stop the flow of the refrigerant through the second gas bypass pipe, wherein the heat source side heat exchangers are adjacently arranged in a top-bottom direction of the apparatus, and wherein the air-conditioning apparatus (100, 200) comprises a controller which is configured to subject, while the heating operation and the defrosting operation are simultaneously performed, the heat source side heat exchanger at a lower level to the defrosting operation and then to subject the heat source side heat exchanger at an upper level to the defrosting operation, wherein a defrosting mode refrigerant decrease detecting unit is configured to detect an amount of refrigerant circulated through the main circuit, wherein the air-conditioning apparatus (100, 200) is configured to control opening and closing of the second opening and closing device in accordance with a detection result of the defrosting mode refrigerant decrease detecting unit. a main circuit including a compressor, a load side heat exchanger, a load side expansion device, a plurality of heat source side heat exchangers connected in parallel one another, and an accumulator that are connected by refrigerant pipes to form at least a heating circuit; a first gas bypass pipe branching off from the refrigerant pipe on a discharge side of the compressor and allowing refrigerant to flow into a defrosting-target heat source side heat exchanger of the heat source side heat exchangers; a plurality of first opening and closing devices arranged in the first gas bypass pipe, the first opening and closing devices each configured to permit or stop a flow of the refrigerant through the first gas bypass pipe; characterized by a second gas bypass pipe branching off from the refrigerant pipe on the discharge side of the compressor and allowing the refrigerant to flow into the accumulator; at least one second opening and closing device disposed in the second gas bypass pipe, the second opening and closing device configured to permit or stop the flow of the refrigerant through the second gas bypass pipe, wherein the heat source side heat exchangers are adjacently arranged in a top-bottom direction of the apparatus, and wherein the air-conditioning apparatus (100, 200) comprises a controller which is configured to subject, while the heating operation and the defrosting operation are simultaneously performed, the heat source side heat exchanger at a lower level to the defrosting operation and then to subject the heat source side heat exchanger at an upper level to the defrosting operation, wherein a defrosting mode refrigerant decrease detecting unit is configured to detect an amount of refrigerant circulated through the main circuit, wherein the air-conditioning apparatus (100, 200) is configured to control opening and closing of the second opening and closing device in accordance with a detection result of the defrosting mode refrigerant decrease detecting unit.