PURPOSE: To unify the water content of a base plate and to prevent it from bending, by a method wherein the center area of a felt has the water content diferrent from that on the circumference, and a base plate is placed to be pressed thereon.

CONSTITUTION: Belt 2 is stretched over roller 1 and 1' to be rotated and felts 3 are mounted on this belt 2 in plural, which are moved together with the belt. Water sprayer 8 is used to apply a moisture on the circumference of the felt 3 which is dried out with dryer 7 so that the water content is increased and the water content of the felt is partially differed. Next, base plate 4 manufactured on the other paper process is placed on the felt 3, then the base plate 4 is pressed upon press table 6 with press 5. By arranging this, the water content of the base plate after being pressed is unified, and the bending of the base plate due to non-uniformity of the water content is prevented.