A method for measuring the concentration of creatinine includes the steps of: (A) mixing a sample containing creatinine with a creatinine quantitative reagent including 1-methoxy-5-methylphenazinium in the absence of picric acid and any enzyme responsive to creatinine, to cause the creatinine to reduce the 1-methoxy-5-methylphenazinium; (B) electrochemically or optically measuring the amount of the 1-methoxy-5-methylphenazinium reduced in the step (A); and (C) determining the concentration of the creatinine contained in the sample from the amount of the reduced 1-methoxy-5-methylphenazinium measured in the step (B).