The analyzing method for quantifying urea in a sample solution includes: a pretreatment step of pretreating the sample solution with at least one of a membrane device including a reverse osmosis membrane and an ion exchange device including an ion exchanger; and an analyzing step of analyzing a target substance in the pretreated sample solution. The analyzing step is based on, for example, flow injection analysis (FIA), and includes a step of quantifying the target substance by measuring the absorbance of a liquid containing a substance generated by reacting the target substance with a reagent.