[Problem] A stable liquid-form pharmaceutical composition wherein the decomposition with time is prevented and a stabilization method thereof is provided, for providing a liquid-form pharmaceutical composition of 1-(2-methoxyethyl)-2-methyl-4,9-dioxo-3-(pyrazin-2-ylmethyl)-4,9-dihydro-1H-naphtho[2,3-d]imidazol-3-ium or a pharmaceutically acceptable salt thereof to a clinical field.[Means for Solution] Related are a stable pharmaceutical composition of 1-(2-methoxyethyl)-2-methyl-4,9-dioxo-3-(pyrazin-2-ylmethyl)-4,9-dihydro-1H-naphtho[2,3-d]imidazol-3-ium or a pharmaceutically acceptable salt thereof, containing one kind or two or more kinds selected from the group consisting of carbazochrome sodium sulfonate and derivatives thereof, vitamin C's, butylhydroxylanisol, vitamin E's, Vitamin P's, gallic acid, propyl gallate, alpha-thioglycerine and cysteine hydrochloride, as well as a lyophilized product thereof and a stabilizing method thereof.