To dispense with a back sealer coating step and a subsequent drying step, and to obtain a building plate which effectively exhibits a waterproofing function, by backing a plastic sheet to a cement bedrock ground.

After clear coating, while a plate temperature (surface temperature) at an outlet of a drier is measured with a noncontact surface thermometer at the time of drying, heat treatment conditions of a final chamber in the drier are controlled. For instance, the plate temperature immediately after being taken out of the drier is controlled to 140±2°C, and the coated plate is quickly subjected to thermocompression bonding (held at a pinching pressure of, e.g. to 330±50 kg/m2 over 5 sec) as it is. Then, the plate is quenched to about 90°C to harden a polyethylene sheet 4, followed by carrying out cooling of the same. Thereafter, the surplus polyethylene sheet 4 protruded from a contour boundary of a cement bedrock is cut off. Thus, a back coating step and a drying step are dispensed with.