PURPOSE: To fix a floor board to a foam finished ground flooring surely without showing any working mark.

CONSTITUTION: A small hole 12 penetrating to the inside of a floor joist 4 of a foam finished ground flooring 3 from the surface of a floor board 5 is formed, and an adhesive 14 is poured into the small hole 12 and, at the same time, a core 15 is inserted thereinto. The adhesive 14 is solidified to fix the floor board 5 and foam finished ground flooring 3 are fixed to each other through the core 15. Even if the finished ground flooring 3 is formed of a foam hard to adhere, the floor board 5 can be surely fixed to the finished ground flooring 3, and since the small hole 12 is small-sized enough to pour the material thereinto, the floor is so finished that no working mark is conspicuous.