Abstract: A heating medium heating device includes: a plate-shaped PTC heater formed by making compressive heat transfer sheets cover individually two surfaces of a PTC element; a first heating medium distribution box including a first matching surface in which a PTC heater accommodating recess is formed, the PTC heater accommodating recess including a bottom surface with which the compressive heat transfer sheet located on a first-surface side of the PTC heater is in close contact; a second heating medium distribution box including a flat second matching surface that closes the PTC heater accommodating recess by being bonded, liquid-tightly via a liquid gasket, to the first matching surface, the second matching surface being a surface with which the compressive heat transfer sheet located on a second-surface side of the PTC heater is in close contact; and a barrier part rising from a peripheral edge part of the PTC heater towards the second matching surface.