PURPOSE: To easily and efficiently produce a good joint part by sticking a non-water-absorptive or low-water-absorptive surface-dressed board on a backing board, so that the shrinkage due to hardening of a joint material at the time of manufacture of a panel can be prevented.

CONSTITUTION: A low-water-absorptive or non-water-absorptive surface-dressed board 2 is stuck on the surface of a backing board 1, and a joint, material 3 is filled in joint gaps, and then the joint material 3 is pressed by a water- absorptive free roller 6, so that the water inside the joint material 3 is absorbed by the water-absorptive free roller 6. Thereby, the joint material 3 can be closely and densely filled in the joint gaps, and the occurrence of shrinkage due to hardening can be prevented. Thus, a good joint part 5 can be easily and efficiently formed.