PURPOSE: To color a plastic board, on which a signal recording part for reading out an information signal optically is formed, such that the type, the use, and the like can be determined instantaneously while sustaining good optical characteristics by admixing a coloring matter having particle size of specific μm or less.

CONSTITUTION: A solvent and a dye of red or blue, for example, are circulated through a filter, e.g. a filter paper or a screen of 325 mesh, using methylene chloride as an extraction liquid thus removing foreign matters and impurities. Crystal particles of that purified dye is then dried to obtain a dye from which methylene chloride is removed and the dye is mixed with an acryl based plasticizer. The mixture is fed through a triple roll thus dispersing the particles while pulverizing. It is filtered through a 200 mesh screen and a polycarbonate board 2 is injection molded using a coloring matter having maximum particle size of 50μm or less. Finally, irregular pits 2a and a reflective layer 3 are formed on the board 2 and covered with a protective layer 4 of UV-curing resin. Consequently, an optical information recording medium having good optical characteristics can be obtained and the type, use, etc., thereof can be determined instantaneously by coloring.