To reduce the number of part items for a hot water control mechanism by providing a cam part on the end part protruded out of a case of the driving shaft of an air mixing door, and extending a cam follower on the valve shaft of a water valve to be slid on the cam part.

A cylindrical cam part 23 is concentrically protruded on the shaft end part protruded out of a case of the driving shaft 20 of an air mix door 6. A water valve 25 for controlling the hot water supply to a heater core is mounted on the case side wall outer surface near the cam part 23. The end part of the valve shaft 26 of the water valve 25 is bent and extended in crank form to form a cam follower 27 to be slidingly engaged with the cam surface of the cam part 23. Since this hot water control mechanism 29 has the structure of directly sliding the end of the cam follower 27 extended on the end of the valve shaft 26 of the water valve 25, the number of part items can be reduced as much as possible.