In a heating-medium heating unit equipped with a first heating-medium circulation box and a second heating-medium circulation box which are in close contact with both surfaces of a PTC heater, in which heating-medium circulation passages are formed in the interior, and which are joined to each other in a liquidtight manner, wherein joining surfaces are sealed with liquid gaskets, the heating-medium circulation passages are provided with joining-surface cooling channels that cool the joining surface which is sealed with a liquid gasket and on which the heat from the PTC heater acts. The joining-surface cooling channels are provided at positions closer to the joining surface than to the PTC heater.