A binary refrigeration cycle system comprising: a low temperature-side refrigeration cycle including a heat source-side heat exchanger that is configured to absorb heat from an external heat source, and a low temperature-side compressor; a high temperature-side refrigeration cycle including a utilization-side heat exchanger that is configured to supply heat to a utilization side, and a high temperature-side compressor; an intermediate heat exchanger that is configured to exchange heat between a refrigerant in the low temperature-side refrigeration cycle and a refrigerant in the high temperature-side refrigeration cycle; a casing to which at least the utilization-side heat exchanger is mounted; a utilization-side pipe that is provided for the casing and connected to the utilization-side heat exchanger so as to exchange heat between a circulated utilization-side fluid and the refrigerant in the high temperature-side refrigeration cycle and supply the heat to the utilization side; a bypass passage that is connected to the utilization-side pipe in parallel with the utilization-side heat exchanger so as to feed the utilization-side fluid in the utilization-side pipe from a utilization-side heat exchanger outlet side to a utilization-side heat exchanger inlet side; and a fluid control unit that is configured to control a flow of the utilization-side fluid circulated in the bypass passage, wherein the fluid control unit includes a utilization-side fluid temperature detection unit that is configured to detect a temperature of the utilization-side fluid flowing into the utilization-side heat exchanger, an external heat source temperature detection unit that is provided at the heat source-side heat exchanger to detect a temperature of the external heat source, and a flow control valve that is configured to change a flow rate in the bypass passage, characterized in that the fluid control unit is controlled so as to open the flow control valve when a difference between the temperature of the utilization-side fluid detected by the utilization-side fluid temperature detection unit and the temperature of the external heat source detected by the external heat source temperature detection unit is equal to or smaller than a predetermined value. a low temperature-side refrigeration cycle including a heat source-side heat exchanger that is configured to absorb heat from an external heat source, and a low temperature-side compressor; a high temperature-side refrigeration cycle including a utilization-side heat exchanger that is configured to supply heat to a utilization side, and a high temperature-side compressor; an intermediate heat exchanger that is configured to exchange heat between a refrigerant in the low temperature-side refrigeration cycle and a refrigerant in the high temperature-side refrigeration cycle; a casing to which at least the utilization-side heat exchanger is mounted; a utilization-side pipe that is provided for the casing and connected to the utilization-side heat exchanger so as to exchange heat between a circulated utilization-side fluid and the refrigerant in the high temperature-side refrigeration cycle and supply the heat to the utilization side; a bypass passage that is connected to the utilization-side pipe in parallel with the utilization-side heat exchanger so as to feed the utilization-side fluid in the utilization-side pipe from a utilization-side heat exchanger outlet side to a utilization-side heat exchanger inlet side; and a fluid control unit that is configured to control a flow of the utilization-side fluid circulated in the bypass passage, wherein the fluid control unit includes a utilization-side fluid temperature detection unit that is configured to detect a temperature of the utilization-side fluid flowing into the utilization-side heat exchanger, an external heat source temperature detection unit that is provided at the heat source-side heat exchanger to detect a temperature of the external heat source, and a flow control valve that is configured to change a flow rate in the bypass passage, characterized in that the fluid control unit is controlled so as to open the flow control valve when a difference between the temperature of the utilization-side fluid detected by the utilization-side fluid temperature detection unit and the temperature of the external heat source detected by the external heat source temperature detection unit is equal to or smaller than a predetermined value.