Provided are: a heat medium heating device capable of bringing: flat heat exchange tubes and PTC heaters; and inlet/outlet header parts of the flat heat exchange tubes, into sufficiently close contact with each other, and incorporating these components into a casing; and a vehicular air-conditioning device including the same. In a heat medium heating device, flat heat exchange tubes each including a flat tube part and inlet/outlet header parts and PTC heaters are stacked in layers, and a heat exchange holding member presses the flat heat exchange tubes from one side, whereby the stacked structure is incorporated on an inner bottom surface of a casing. In this configuration, at least uppermost one of the flat heat exchange tubes has one surface pressed by the heat exchange holding member, the one surface being formed into a planar shape in which the flat tube part and the inlet/outlet header parts are flattened.