A heat source system (10, 110), comprising: a refrigerant circuit formed by connecting a compressor, a heat source-side heat exchanger, an expansion valve, and a load-side heat exchanger, the compressor being configured to compress refrigerant, the heat source-side heat exchanger being configured to exchange heat between air and the refrigerant, the expansion valve being configured to control a flow rate of the refrigerant, the load-side heat exchanger being configured to exchange heat between heat medium and the refrigerant; a heat medium circuit (40, 140) including a flow control pump configured to pump the heat medium to the load-side heat exchanger, and configured to circulate the heat medium to a load device that is provided downstream of the load-side heat exchanger; and a controller (21, 121) configured to control a frequency of the flow control pump and to carry out a freezing prevention control, the controller (21, 121) including a storage unit storing an increase amount table in which the frequency of the flow control pump and a frequency increase amount that is an amount by which the frequency of the flow control pump is increased are associated with each other, wherein the storage unit also stores a plurality of flow rate thresholds that are set in association with the frequency of the flow control pump and wherein the flow rate thresholds are set to a temperature at which the heat medium flowing through the load-side heat exchanger may be frozen unless the controller executes the freezing prevention control, when an evaporating temperature of the load-side heat exchanger drops to the flow rate threshold for the current frequency of the flow control pump, the controller (21, 121) being configured to carry out the freezing prevention control and to refer to the increase amount table with the frequency of the flow control pump to determine the frequency increase amount, and increase the frequency of the flow control pump by the frequency increase amount that is determined. a refrigerant circuit formed by connecting a compressor, a heat source-side heat exchanger, an expansion valve, and a load-side heat exchanger, the compressor being configured to compress refrigerant, the heat source-side heat exchanger being configured to exchange heat between air and the refrigerant, the expansion valve being configured to control a flow rate of the refrigerant, the load-side heat exchanger being configured to exchange heat between heat medium and the refrigerant; the compressor being configured to compress refrigerant, the heat source-side heat exchanger being configured to exchange heat between air and the refrigerant, the expansion valve being configured to control a flow rate of the refrigerant, the load-side heat exchanger being configured to exchange heat between heat medium and the refrigerant; a heat medium circuit (40, 140) including a flow control pump configured to pump the heat medium to the load-side heat exchanger, and configured to circulate the heat medium to a load device that is provided downstream of the load-side heat exchanger; and a controller (21, 121) configured to control a frequency of the flow control pump and to carry out a freezing prevention control, the controller (21, 121) including a storage unit storing an increase amount table in which the frequency of the flow control pump and a frequency increase amount that is an amount by which the frequency of the flow control pump is increased are associated with each other, wherein the storage unit also stores a plurality of flow rate thresholds that are set in association with the frequency of the flow control pump and wherein the flow rate thresholds are set to a temperature at which the heat medium flowing through the load-side heat exchanger may be frozen unless the controller executes the freezing prevention control, when an evaporating temperature of the load-side heat exchanger drops to the flow rate threshold for the current frequency of the flow control pump, the controller (21, 121) being configured to carry out the freezing prevention control and to refer to the increase amount table with the frequency of the flow control pump to determine the frequency increase amount, and increase the frequency of the flow control pump by the frequency increase amount that is determined.