A method for generating a character string of fixed or variable length which is substantially unique with respect to a chemical structural formula, on the basis of the kind of each atom constituting the compound and the bonding relation of the atoms and/or the kind of isomers, and employing the character string as an ID symbol of the said compound, and a method for converting the above-mentioned character string to data or a character string of smaller data size by using a character string converting function and employing it as the ID symbol of the said compound.