A hot-water supply system comprising: a heat source device; a hot-water-stored-amount detecting means (21a,21b,...21n) configured to detect an amount of hot water stored in a hot-water storage tank configured to store hot water heated by the heat source device; an electrical valve that is provided in a path of supplying the hot water heated by the heat source device to the hot-water storage tank and that is configured to adjust a flow level of hot water supplied to the hot-water storage tank according to a control signal; a controlling means that is provided in the heat source device and that is configured to control the heat source device based on information about the amount of stored hot water detected by the hot-water-stored-amount detecting means (21a,21b,...21n)and based on a degree-of-opening signal of the electrical valve; and an interface means that is connected to the hot-water-stored-amount detecting means (21a,21b,...21n) and the electrical valve via electrical wires (5a,5b) and that is also connected to the controlling means via a communication line, the interface means converts the information about the amount of stored hot water which is detected by the hot-water-stored-amount detecting means (21a,21b,...21n) and the degree-of-opening signal of the electrical valve input via the electrical wires (5a,5b) into signals that can be sent to and received from the controlling means and outputs these signals to the controlling means via the communication line, and converts the control signal obtained from the controlling means via the communication line into a signal that can be sent to and received from the electrical valve and outputs the signal to the electrical valve via the electrical wire, wherein the controlling means includes a first connecting means configured to allow connection with the interface means so that information can be sent and received via the communication line. a heat source device; a hot-water-stored-amount detecting means (21a,21b,...21n) configured to detect an amount of hot water stored in a hot-water storage tank configured to store hot water heated by the heat source device; an electrical valve that is provided in a path of supplying the hot water heated by the heat source device to the hot-water storage tank and that is configured to adjust a flow level of hot water supplied to the hot-water storage tank according to a control signal; a controlling means that is provided in the heat source device and that is configured to control the heat source device based on information about the amount of stored hot water detected by the hot-water-stored-amount detecting means (21a,21b,...21n)and based on a degree-of-opening signal of the electrical valve; and an interface means that is connected to the hot-water-stored-amount detecting means (21a,21b,...21n) and the electrical valve via electrical wires (5a,5b) and that is also connected to the controlling means via a communication line, the interface means converts the information about the amount of stored hot water which is detected by the hot-water-stored-amount detecting means (21a,21b,...21n) and the degree-of-opening signal of the electrical valve input via the electrical wires (5a,5b) into signals that can be sent to and received from the controlling means and outputs these signals to the controlling means via the communication line, and converts the control signal obtained from the controlling means via the communication line into a signal that can be sent to and received from the electrical valve and outputs the signal to the electrical valve via the electrical wire, wherein the controlling means includes a first connecting means configured to allow connection with the interface means so that information can be sent and received via the communication line.