PROBLEM TO BE SOLVED: To provide a measurement method having a wider determination range in a measurement system of urea nitrogen and to obtain a more inexpensive reagent. SOLUTION: This measurement method comprises reacting urea with urease in a liquid phase containing at least two kinds of buffer solutions in the presence of a pH indicator and optically detecting change in pH of the liquid phase caused by formed ammonia. This reagent comprises a first reagent composed of at least two kinds of buffer solutions containing at least a pH indicator and a second reagent composed of a solution containing at least urease.
