To provide a method for purifying a solution containing a specified anthrapyridone compound treated with an alkali or its salt expressed as an exemplified compound 1, etc.

This method for purifying the solution containing the specified anthrapyridone compound treated with the alkali or its salt comprises, for example, decreasing a pH of a solution containing the exemplified compound 1, by adding an acid, such as nitric acid and sulfuric acid, to the solution, precipitating an exemplified compound 2, and filtering the precipitated compound, so as to purify the exemplified compound 1.