PURPOSE: To obtain the compound in a high purity by adding an equivalent water to a guanidine compound and an organoboron compound such as an alkoxyboron in an organic solvent and reacting these components in a nonaqueous solution system.

CONSTITUTION: An equivalent water is added to a guanidine compound of the formula (R1 to R4 are H, an alkyl, aryl, alkylaryl or hydroxyalkyl with the proviso that R1 may be amide) (especially guanidine, 1-methylguanidine or 1,1- dimethylguanidine) and an organoboron compound (e.g. butoxyboronic acid) in an organic solvent such as butanol and these components are made to react in a nonaqueous solution system to provide the boric acid salt of guanidine compound. Since the compound having a high purity is obtained by this method, purification process is made unnecessary and production at a low cost is made possible.