In a heat medium heating device 10 in which a heat exchange element 20 formed by stacking multiple flat heat exchange tubes 21 and multiple pairs of PTC heaters 26 in multiple layers is fastened and fixed into a casing 11 including heat medium outlet and inlet paths 15 and 16, connection portions 48 and 49 between a power supply HV harness and a control LV harness 44 are provided on an outer surface on one surface side where the heat medium outlet and inlet paths 15 and 16 of the casing 11 are provided, and a control substrate 33 that controls energization to the PTC heaters 26 is disposed on an inside on the same side as a surface on which the connection portions 48 and 49 of the casing 11 is provided.