An air-conditioning apparatus comprising: a refrigerant circuit through which refrigerant circulates; a heat medium circuit through which a heat medium circulates; an outdoor unit located in the refrigerant circuit and configured to exchange heat between the refrigerant and outdoor air; a plurality of indoor units located in the heat medium circuit and configured to exchange heat between the heat medium and indoor air to condition the indoor air; and an intermediate unit located in the refrigerant circuit and the heat medium circuit and configured to exchange heat between the refrigerant and the heat medium and send the heat medium subjected to heat exchange with the refrigerant to the plurality of indoor units, each of the plurality of indoor units including a flow control device configured to regulate a flow rate of the heat medium leaving a corresponding one of the plurality of indoor units, and an indoor controller configured to adjust an opening degree of the flow control device, the intermediate unit including an intermediate heat exchanger configured to exchange heat between the refrigerant and the heat medium, a circulation device configured to circulate the heat medium between the intermediate heat exchanger and each of the plurality of indoor units, and an intermediate controller configured to control the circulation device, characterized in that the intermediate controller is configured to, when at least one indoor unit of the plurality of indoor units starts operating, determine whether a flow rate of the heat medium flowing into the intermediate heat exchanger from the plurality of indoor units is greater than or equal to a predetermined minimum flow rate, and in response to determining that the flow rate is less than the minimum flow rate, transmit an open instruction signal to the indoor controller of the indoor unit that is in a non-operation state of the plurality of indoor units, the open instruction signal representing an instruction to increase the opening degree of the flow control device, and wherein the indoor controller is configured to, when receiving the open instruction signal, increase the opening degree of the flow control device in response to the open instruction signal. a refrigerant circuit through which refrigerant circulates; a heat medium circuit through which a heat medium circulates; an outdoor unit located in the refrigerant circuit and configured to exchange heat between the refrigerant and outdoor air; a plurality of indoor units located in the heat medium circuit and configured to exchange heat between the heat medium and indoor air to condition the indoor air; and an intermediate unit located in the refrigerant circuit and the heat medium circuit and configured to exchange heat between the refrigerant and the heat medium and send the heat medium subjected to heat exchange with the refrigerant to the plurality of indoor units, each of the plurality of indoor units including a flow control device configured to regulate a flow rate of the heat medium leaving a corresponding one of the plurality of indoor units, and an indoor controller configured to adjust an opening degree of the flow control device, a flow control device configured to regulate a flow rate of the heat medium leaving a corresponding one of the plurality of indoor units, and an indoor controller configured to adjust an opening degree of the flow control device, the intermediate unit including an intermediate heat exchanger configured to exchange heat between the refrigerant and the heat medium, a circulation device configured to circulate the heat medium between the intermediate heat exchanger and each of the plurality of indoor units, and an intermediate controller configured to control the circulation device, characterized in that the intermediate controller is configured to, when at least one indoor unit of the plurality of indoor units starts operating, determine whether a flow rate of the heat medium flowing into the intermediate heat exchanger from the plurality of indoor units is greater than or equal to a predetermined minimum flow rate, and in response to determining that the flow rate is less than the minimum flow rate, transmit an open instruction signal to the indoor controller of the indoor unit that is in a non-operation state of the plurality of indoor units, the open instruction signal representing an instruction to increase the opening degree of the flow control device, and an intermediate heat exchanger configured to exchange heat between the refrigerant and the heat medium, a circulation device configured to circulate the heat medium between the intermediate heat exchanger and each of the plurality of indoor units, and an intermediate controller configured to control the circulation device, characterized in that the intermediate controller is configured to, when at least one indoor unit of the plurality of indoor units starts operating, determine whether a flow rate of the heat medium flowing into the intermediate heat exchanger from the plurality of indoor units is greater than or equal to a predetermined minimum flow rate, and in response to determining that the flow rate is less than the minimum flow rate, transmit an open instruction signal to the indoor controller of the indoor unit that is in a non-operation state of the plurality of indoor units, the open instruction signal representing an instruction to increase the opening degree of the flow control device, and wherein the indoor controller is configured to, when receiving the open instruction signal, increase the opening degree of the flow control device in response to the open instruction signal.