PURPOSE: To enable a difference in heights appeared on a surface finishing material by a method wherein a ground material having a substantial equal thickness to that of the heater device is mounted on the upper surface of the base plywood around the heater device.

CONSTITUTION: A plurality of stages of base plywoods 2 are arranged on the upper surface of a particle board 1. The base plywood 2 is provided with a recess 6 made by a cutter or the like to which the terminal part 4 or cable part 5 for the heater device 3 is fitted. A plate-like heater device 3 is mounted on the upper surface of the base plywood 2. A plurality of felt-like substrate material 7 having substantial same thickness as that of the heater device 3 are arranged on the upper surface of the base plywood 2. Lastly, the substrate plate 7 and the upper surface of the heater device 3 are covered with the surface finishing carpet 8.