A controller to control an air conditioning apparatus configured to operate in operation modes including a heating mode and a defrosting mode, the air conditioning apparatus comprising: a compressor configured to compress a first heat medium; a first heat exchanger configured to exchange heat between the first heat medium and outdoor air; a second heat exchanger configured to exchange heat between the first heat medium and a second heat medium; a plurality of third heat exchangers (31, 41, 51) each configured to exchange heat between the second heat medium and indoor air; a plurality of flow rate control valves (33, 43, 53) each configured to control a flow rate of the second heat medium flowing through a corresponding one of the plurality of third heat exchangers; and a pump configured to circulate the second heat medium between the plurality of third heat exchangers (31, 41, 51) and the second heat exchanger, wherein the controller is configured to, in the heating mode, open the flow rate control valve corresponding to a heat exchanger, of the plurality of third heat exchangers (31, 41, 51), to which a request for air conditioning has been made, and to close the flow rate control valve corresponding to a heat exchanger, of the plurality of third heat exchangers (31, 41, 51), to which the request for air conditioning has not been made, and the controller is configured to, in the defrosting mode, when a temperature of the second heat medium is lower than a first determination temperature, open the flow rate control valve corresponding to the heat exchanger to which the request for air conditioning has not been made wherein the controller is configured to, in the defrosting mode, when the temperature of the second heat medium is higher than a second determination temperature, close the flow rate control valve corresponding to the heat exchanger to which the request for air conditioning has not been made, and the second determination temperature is a temperature higher than or equal to the first determination temperature. a compressor configured to compress a first heat medium; a first heat exchanger configured to exchange heat between the first heat medium and outdoor air; a second heat exchanger configured to exchange heat between the first heat medium and a second heat medium; a plurality of third heat exchangers (31, 41, 51) each configured to exchange heat between the second heat medium and indoor air; a plurality of flow rate control valves (33, 43, 53) each configured to control a flow rate of the second heat medium flowing through a corresponding one of the plurality of third heat exchangers; and a pump configured to circulate the second heat medium between the plurality of third heat exchangers (31, 41, 51) and the second heat exchanger, wherein the controller is configured to, in the heating mode, open the flow rate control valve corresponding to a heat exchanger, of the plurality of third heat exchangers (31, 41, 51), to which a request for air conditioning has been made, and to close the flow rate control valve corresponding to a heat exchanger, of the plurality of third heat exchangers (31, 41, 51), to which the request for air conditioning has not been made, and the controller is configured to, in the defrosting mode, when a temperature of the second heat medium is lower than a first determination temperature, open the flow rate control valve corresponding to the heat exchanger to which the request for air conditioning has not been made wherein the controller is configured to, in the defrosting mode, when the temperature of the second heat medium is higher than a second determination temperature, close the flow rate control valve corresponding to the heat exchanger to which the request for air conditioning has not been made, and the second determination temperature is a temperature higher than or equal to the first determination temperature.