The material will absorb moisture to expand when the humidity in the air is high, whereas it will shrink when it is dry. Particularly when floor heating is used, the floor may dry and shrink to create the space at the joint of the floor. This is an unavoidable phenomenon as it uses living materials, and the floor gradually extends and the gap becomes smaller when the floor heating is stopped using. Conversely, the floor may stretch and the seam portion may be pushed up.