The present invention provides a composition containing a benzamidine derivative that does not decompose even when placed under humidification conditions and a method for stabilizing a benzamidine derivative. According to the present invention, decomposition reaction of a benzamidine derivative can be suppressed by adding to a benzamidine derivative represented by general formula (I) or a pharmacologically acceptable salt thereof, at least one type of electrolyte selected from the group consisting of halide salts of alkaline metal or alkaline earth metal and alkaline metal salts or alkaline earth metal salts of perchloric acid. ##STR00001##