To be able to control minute flow in a heat exchanger flow control valve without requiring a minute opening portion, an opening surface area of a hot-water inlet pipe into which hot water flows from an engine is restricted by a first restricting portion of a control passage provided in a needle valve of a flow control valve, and along with this, an opening surface area of a hot-water outlet pipe connected to an inlet side of a heat exchanger for heater use is restricted by a second restricting portion of a control passage formed in the needle valve, and a middle portion of these two control passages is constantly connected to a bypass circuit by a third restricting portion. Pressure of the middle portion thereof can thereby be lowered, pressure applied to the heat exchanger can be adequately reduced, and for this reason minute flow through the valve can be controlled even without establishing a minute opening portion using the needle valves Also, change in heat-exchanger hot-water flow can be reduced by structuring the first and second restricting portions in a configuration whereby flow constant is reduced as amount of flow increases and, along with this, structuring the third restricting portion in a configuration whereby flow constant is maintained substantially uniformly even if amount of flow increases.