A hot-water supply device comprising: a first flowing path through which a first heat medium flows; a second flowing path through which a second heat medium flows from a water supply source toward a hot water supply terminal; a heat exchanger in which heat of the first heat medium flowing through the first flowing path and heat of the second heat medium flowing through the second flowing path are exchanged with each other; fluid transfer means provided in the first flowing path to flow the first heat medium through the first flowing path; flow rate detecting means provided in the second flowing path; hot water supply temperature detecting means provided in the second flowing path between the heat exchanger and the hot water supply terminal; and control means which is configured for controlling the fluid transfer means to carry out a hot water supply operation such that hot water supply temperature detected by the hot water supply temperature detecting means becomes equal to set temperature, wherein the device is configured such that when the flow rate detecting means detects that a flow rate of the second heat medium is equal to a predetermined flow rate or more, the control means actuates the fluid transfer means and starts the hot water supply operation, and the control means estimates a held heat amount of the heat exchanger when the flow rate detecting means detects that the flow rate is equal to the predetermined flow rate or more, and the control means controls the fluid transfer means such that the relatively larger the held heat amount is, the smaller a transfer flow rate of the first heat medium when the hot water supply temperature is brought close to the set temperature becomes, characterized in that the device if configured such that when the flow rate detecting means detects the flow rate less than the predetermined flow rate, the control means stops the fluid transfer means to complete the hot water supply operation, the control means includes timer means which measures time elapsed from an instant when a last hot water supply operation is completed to an instant when a next hot water supply operation is started, and the control means estimates the held heat amount by the elapsed time when the flow rate detecting means detects a flow rate not less than the predetermined flow rate and the hot water supply operation is restarted, and when the elapsed time is relatively short, the control means determines that the held heat amount is relatively large. a first flowing path through which a first heat medium flows; a second flowing path through which a second heat medium flows from a water supply source toward a hot water supply terminal; a heat exchanger in which heat of the first heat medium flowing through the first flowing path and heat of the second heat medium flowing through the second flowing path are exchanged with each other; fluid transfer means provided in the first flowing path to flow the first heat medium through the first flowing path; flow rate detecting means provided in the second flowing path; hot water supply temperature detecting means provided in the second flowing path between the heat exchanger and the hot water supply terminal; and control means which is configured for controlling the fluid transfer means to carry out a hot water supply operation such that hot water supply temperature detected by the hot water supply temperature detecting means becomes equal to set temperature, wherein the device is configured such that when the flow rate detecting means detects that a flow rate of the second heat medium is equal to a predetermined flow rate or more, the control means actuates the fluid transfer means and starts the hot water supply operation, and the control means estimates a held heat amount of the heat exchanger when the flow rate detecting means detects that the flow rate is equal to the predetermined flow rate or more, and the control means controls the fluid transfer means such that the relatively larger the held heat amount is, the smaller a transfer flow rate of the first heat medium when the hot water supply temperature is brought close to the set temperature becomes, characterized in that the device if configured such that when the flow rate detecting means detects the flow rate less than the predetermined flow rate, the control means stops the fluid transfer means to complete the hot water supply operation, the control means includes timer means which measures time elapsed from an instant when a last hot water supply operation is completed to an instant when a next hot water supply operation is started, and the control means estimates the held heat amount by the elapsed time when the flow rate detecting means detects a flow rate not less than the predetermined flow rate and the hot water supply operation is restarted, and when the elapsed time is relatively short, the control means determines that the held heat amount is relatively large.