An air-conditioning apparatus comprising: a refrigerant circuit through which a heat source side refrigerant is circulated, the refrigerant circuit including a compressor, a heat source side heat exchanger, a plurality of expansion devices, a controller for controlling the operation of the air-conditioning apparatus, means for detecting a heat medium temperature and refrigerant passages of a plurality of intermediate heat exchangers which are connected by refrigerant pipes; and a heat medium circuit through which a heat medium is circulated, the heat medium circuit including a plurality of pumps, a plurality of indoor units each including a use side heat exchanger and an air sending device, and heat medium passages of the intermediate heat exchangers which are connected by heat medium conveying pipes, wherein the intermediate heat exchangers are configured to exchange heat between the heat source side refrigerant and the heat medium, characterised in that the controller is configured to switch the apparatus from an operation mode in which all of a plurality of indoor units each including the use side heat exchanger and the air-sending device, the air-sending device being configured to supply air to the use side heat exchanger, are in non-operation to another operation mode in which the controller instructs at least one of the indoor units to start a cooling operation mode or a heating operation mode, the heat medium conveyed to the use side heat exchanger included in the indoor unit which has received a start instruction is cooled or heated until a predetermined time, which is estimated as time the heat medium takes to reach a predetermined temperature, elapsed, the predetermined time being assumed from a total volume of the heat medium in the heat medium circuit, and after that, the controller actuates the air-sending device included in the indoor unit which starts the cooling operation mode or the heating operation mode. a refrigerant circuit through which a heat source side refrigerant is circulated, the refrigerant circuit including a compressor, a heat source side heat exchanger, a plurality of expansion devices, a controller for controlling the operation of the air-conditioning apparatus, means for detecting a heat medium temperature and refrigerant passages of a plurality of intermediate heat exchangers which are connected by refrigerant pipes; and a heat medium circuit through which a heat medium is circulated, the heat medium circuit including a plurality of pumps, a plurality of indoor units each including a use side heat exchanger and an air sending device, and heat medium passages of the intermediate heat exchangers which are connected by heat medium conveying pipes, wherein the intermediate heat exchangers are configured to exchange heat between the heat source side refrigerant and the heat medium, characterised in that the controller is configured to switch the apparatus from an operation mode in which all of a plurality of indoor units each including the use side heat exchanger and the air-sending device, the air-sending device being configured to supply air to the use side heat exchanger, are in non-operation to another operation mode in which the controller instructs at least one of the indoor units to start a cooling operation mode or a heating operation mode, the heat medium conveyed to the use side heat exchanger included in the indoor unit which has received a start instruction is cooled or heated until a predetermined time, which is estimated as time the heat medium takes to reach a predetermined temperature, elapsed, the predetermined time being assumed from a total volume of the heat medium in the heat medium circuit, and after that, the controller actuates the air-sending device included in the indoor unit which starts the cooling operation mode or the heating operation mode.