To obtain a floor heating method utilizing night power by combining a floor set panel unit with the integration of a latent heat accumulation body and a planar electric heating resistor and sensible heat of a concrete heat accumulation layer.

A panel unit has a hydrate based latent heat accumulation body which has a proper melting temperature for floor heating and a large amount of latent heat at a melting point with excellent thermal histeresis for a long time in terms of the quantity of heat per unit heat accumulation material of 50 Kcal-62 Kcal/Kg and the unit heat capacity of 75 Kcal or more and a planar electric resistance heating body 2 of a heater using a heating method necessary for the melting of the accumulation body as an electric member attached within a range from the maximum height of 200 mm to the minimum height of 12 mm of a wood box-shaped exterior case, which is laminated on the heat accumulation body flush vertically therebetween. The panel unit is combined with sensible heat of the ground concrete and a mortar heat accumulation layer. The result is a smaller size, a thin designing and a handy installation with higher heat accumulating/radiating property.