A controller that controls an air conditioning system, the air conditioning system 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) configured to exchange heat between the second heat medium and indoor air; a plurality of flow rate control valves (33, 43, 53) configured to control flow rates of the second heat medium flowing respectively in the plurality of third heat exchangers (31, 41, 51); a pump configured to circulate the second heat medium between the plurality of third heat exchangers (31, 41, 51) and the second heat exchanger; a plurality of first temperature sensors (34, 44, 54) configured to detect temperatures of the second heat medium flowing respectively into the plurality of third heat exchangers (31, 41, 51); and a plurality of second temperature sensors (35, 45, 55) configured to detect temperatures of the second heat medium flowing respectively out of the plurality of third heat exchangers (31, 41, 51), the controller being configured to control the pump and the compressor, and when a state in which a driving voltage of the pump is set to an upper limit voltage and an opening degree of at least one of the plurality of flow rate control valves (33, 43, 53) is set to a maximum opening degree continues for a first determination time period, the controller being configured to raise an operation frequency of the compressor by a first amount of increase at a first time so as to increase an amount of heat exchange by the second heat exchanger, and when a difference between the temperatures detected by the first temperature sensor (34, 44, 54) and the second temperature sensor (35, 45, 55) corresponding to one of the plurality of third heat exchangers (31, 41, 51) provided with the flow rate control valve (33, 43, 53) set to the maximum opening degree is still smaller than a target value after a lapse of a second determination time period since the first time, the controller being configured to further raise the operation frequency of the compressor by a second amount of increase. 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) configured to exchange heat between the second heat medium and indoor air; a plurality of flow rate control valves (33, 43, 53) configured to control flow rates of the second heat medium flowing respectively in the plurality of third heat exchangers (31, 41, 51); a pump configured to circulate the second heat medium between the plurality of third heat exchangers (31, 41, 51) and the second heat exchanger; a plurality of first temperature sensors (34, 44, 54) configured to detect temperatures of the second heat medium flowing respectively into the plurality of third heat exchangers (31, 41, 51); and a plurality of second temperature sensors (35, 45, 55) configured to detect temperatures of the second heat medium flowing respectively out of the plurality of third heat exchangers (31, 41, 51), the controller being configured to control the pump and the compressor, and when a state in which a driving voltage of the pump is set to an upper limit voltage and an opening degree of at least one of the plurality of flow rate control valves (33, 43, 53) is set to a maximum opening degree continues for a first determination time period, the controller being configured to raise an operation frequency of the compressor by a first amount of increase at a first time so as to increase an amount of heat exchange by the second heat exchanger, and when a difference between the temperatures detected by the first temperature sensor (34, 44, 54) and the second temperature sensor (35, 45, 55) corresponding to one of the plurality of third heat exchangers (31, 41, 51) provided with the flow rate control valve (33, 43, 53) set to the maximum opening degree is still smaller than a target value after a lapse of a second determination time period since the first time, the controller being configured to further raise the operation frequency of the compressor by a second amount of increase.