PURPOSE: To perform easy and rapid indexing of the installation position and the size of a board for building in assembly of a unit building by providing a master for graduations where a plurality of line members are stretched and a storage part for a coloring material by means of which the master is marked on a board for building.

CONSTITUTION: A plaster board 1 is formed such that a plaster 1a forms a core and two surfaces and sides are covered with a master 1b for the plaster board. Meanwhile, a device 2 to mark the plaster board 1 with graduation lines comprises a graduation master 3 and a Japanese ink storage part 4 to feed Japanese ink. The graduation master 3 comprises a rectangular machine frame 3a and stretched threads 3b with Japanese ink. The Japanese ink storage part 4 comprises floss silk, i.e. Japanese ink floss silk 4a, and a rectangular floss silk container 4b to contain the Japanese ink floss silk. In marking of the plaster board 1 with longitudinal and lateral lines, the graduation master 3 is first dipped in Japanese ink. Secondly, the plaster board 1 is marked with lines. This device and method perform easy and rapid indexing of the installation position and the size of a board for building.