PURPOSE: To prevent the immersion of water from a connecting part by attaching waterproofed surface plates adhesively on the upper surface of a plate provided with a male-type fitting part and a female type fitting part on both ends, fitting the male and female fitting parts of the adjoining plates respectively and providing the connecting part with a waterproof filler.

CONSTITUTION: A male-type fitting part 1 protruding toward the side and a female-type fitting part 2 recessed inward are respectively provided at both ends of a plate 3, and waterproofed wooden surface plates 4 are adhesively attached on the upper surfaces of the fitting plates 1, 2 so as to form a building plate 5. The male-type fitting part 1 of the adjoining building plate 5a is fitted into the female type fitting part 2 of the other building plate 5b to be connected to each other, and a waterproofing filler 7 is filled into the joint part 6a of a fitted part 6. As a result, excellent waterproofing performance can be obtained enabling also the use of this building plate in water way as well as the immersion of water from the connecting part can be prevented.