PURPOSE: To aim at eliminating the necessity of a melting furnace and coating steps by spraying a powder-like thermally molten type water-proof agent directly on a floor board, and by placing a paving material at a high temperature, directly thereover.

CONSTITUTION: A powder-like thermally molten type water-proof agent is sprayed, as it is, directly on a floor board 1, and a paving material 3 such as asphalt or the like at a high temperature is placed thereover. With this arrangement, it is possible to eliminate coating steps from a melting step to a coating step for the floor board, and accordingly, it is possible to economically and rapidly carry out the water-proof process.