A heating-medium heating unit, comprising: a flat PTC heater; a first heating-medium circulation box in which a plurality of box components (20,21,30) are stacked one on another, which is in close contact with one surface of the PTC heater, and in which a heating-medium circulation passage is formed in the interior; and a second heating-medium circulation box in which a plurality of box components (50,51) are stacked on one another, which is in close contact with the other surface of the PTC heater, in which a heating-medium circulation passage is formed in the interior, and which is joined to the first heating-medium circulation box in a liquidtight manner, wherein a heating medium that circulates through the heating-medium circulation passages (33,54) in the first and second heating-medium circulation boxes (A,B) is heated, in operation, by heat radiated from both surfaces of the PTC heater, characterized in that at least one of a joining surface (M2,M3,M4) between the box components (20,21,30,50,51) that constitute the first heating-medium circulation box and the second heating-medium circulation box and a joining surface between the first heating-medium circulation box and the second heating-medium circulation box is sealed with a liquid gasket; and the heating-medium circulation passage (33,54) of at least the first heating-medium circulation box or the second heating-medium circulation box is provided with a joining-surface cooling channel (C1,C2) that is sealed with the liquid gasket and that cools the joining surface (M1,M3,M4) on which the heat from the PTC heater acts; and a joining surface of the joining surfaces is provided with an outside sealing section (M3a) that seals a space between the heating-medium circulation passage and the outside and a board sealing section (M3b) that seals a space between the heating-medium circulation passage and a portion communicating with a portion accommodating a board for controlling the PTC heater, in which the width of the board sealing section (M3b) is larger than the width of the outside sealing section (M3a). a flat PTC heater; a first heating-medium circulation box in which a plurality of box components (20,21,30) are stacked one on another, which is in close contact with one surface of the PTC heater, and in which a heating-medium circulation passage is formed in the interior; and a second heating-medium circulation box in which a plurality of box components (50,51) are stacked on one another, which is in close contact with the other surface of the PTC heater, in which a heating-medium circulation passage is formed in the interior, and which is joined to the first heating-medium circulation box in a liquidtight manner, wherein a heating medium that circulates through the heating-medium circulation passages (33,54) in the first and second heating-medium circulation boxes (A,B) is heated, in operation, by heat radiated from both surfaces of the PTC heater, at least one of a joining surface (M2,M3,M4) between the box components (20,21,30,50,51) that constitute the first heating-medium circulation box and the second heating-medium circulation box and a joining surface between the first heating-medium circulation box and the second heating-medium circulation box is sealed with a liquid gasket; and the heating-medium circulation passage (33,54) of at least the first heating-medium circulation box or the second heating-medium circulation box is provided with a joining-surface cooling channel (C1,C2) that is sealed with the liquid gasket and that cools the joining surface (M1,M3,M4) on which the heat from the PTC heater acts; and a joining surface of the joining surfaces is provided with an outside sealing section (M3a) that seals a space between the heating-medium circulation passage and the outside and a board sealing section (M3b) that seals a space between the heating-medium circulation passage and a portion communicating with a portion accommodating a board for controlling the PTC heater, in which the width of the board sealing section (M3b) is larger than the width of the outside sealing section (M3a).