PURPOSE: To remove tetramethylammonium by making solution containing tetramethylammonium compound in contact with bacteria which belong to protomonas group capable of utilizing it as a nutrient and decomposes the tetramethylammonium compound.

CONSTITUTION: Solution containing a tetramethylammonium compound is made in contact with bacteria which belong to group capable of utilizing it and decompose the tetramethylammonium compound to remove tetraammonium in the solution. The representing example of said bacteria is protomonas extroquens (FERM-P 9466). In order to turn the solution containing a compound into harmless, a method to add various kinds of nutrients to said solution to probide a culture medium, in which bacteria belonging to protomonas group and utilize and decompose the tetramethlammonium compound are cultured, is applied.