A heat medium cycle system, comprising: a refrigeration cycle circuit through which refrigerant is circulated, the refrigeration cycle circuit including a compressor, a heat-source-side heat exchanger, an expansion device, and a heat medium heat exchanger connected by pipes; a heat medium cycle circuit through which a heat medium is circulated, the heat medium cycle circuit including a pump circulating the heat medium, the heat medium heat exchanger, and a load-side heat exchanger connected by pipes; an inlet temperature sensor configured to measure a temperature of the heat medium at a heat medium inlet of the heat medium heat exchanger; an inlet pressure sensor configured to measure a pressure of the heat medium at the heat medium inlet of the heat medium heat exchanger; an outlet pressure sensor configured to measure a pressure of the heat medium at a heat medium outlet of the heat medium heat exchanger; an evaporating temperature sensor configured to detect an evaporating temperature of the refrigerant in the heat medium heat exchanger; and a controller; characterized in that the controller is configured to, under a first condition where the heat medium is to freeze in the heat medium heat exchanger, obtain a minimum on-state flow rate at which the heat medium is kept from freezing in the heat medium heat exchanger, on a basis of the temperature of the heat medium at the heat medium inlet measured by the inlet temperature sensor and the evaporating temperature of the refrigerant detected by the evaporating temperature sensor, and to control the pump in such a manner that a flow rate of the heat medium estimated on a basis of a pressure difference of the heat medium obtained from a measurement value of the inlet pressure sensor and a measurement value of the outlet pressure sensor meets greater than or equal to the minimum on-state flow rate. a refrigeration cycle circuit through which refrigerant is circulated, the refrigeration cycle circuit including a compressor, a heat-source-side heat exchanger, an expansion device, and a heat medium heat exchanger connected by pipes; a heat medium cycle circuit through which a heat medium is circulated, the heat medium cycle circuit including a pump circulating the heat medium, the heat medium heat exchanger, and a load-side heat exchanger connected by pipes; an inlet temperature sensor configured to measure a temperature of the heat medium at a heat medium inlet of the heat medium heat exchanger; an inlet pressure sensor configured to measure a pressure of the heat medium at the heat medium inlet of the heat medium heat exchanger; an outlet pressure sensor configured to measure a pressure of the heat medium at a heat medium outlet of the heat medium heat exchanger; an evaporating temperature sensor configured to detect an evaporating temperature of the refrigerant in the heat medium heat exchanger; and a controller; characterized in that the controller is configured to, under a first condition where the heat medium is to freeze in the heat medium heat exchanger, obtain a minimum on-state flow rate at which the heat medium is kept from freezing in the heat medium heat exchanger, on a basis of the temperature of the heat medium at the heat medium inlet measured by the inlet temperature sensor and the evaporating temperature of the refrigerant detected by the evaporating temperature sensor, and to control the pump in such a manner that a flow rate of the heat medium estimated on a basis of a pressure difference of the heat medium obtained from a measurement value of the inlet pressure sensor and a measurement value of the outlet pressure sensor meets greater than or equal to the minimum on-state flow rate.