Object: To provide a test reagent for the analyte in a test sample, utilizing the level of agglutination of a particle suspension which suspend insoluble carrier particles carrying a substance for capturing the analyte as an indicator, which reagent does not undergo self-agglutination during storage, and which non-specific agglutination rarely occurs during measurement, as well as to provide a method for the analyte to be measured in a test sample. Means for Solution: The test reagent for the analyte comprises at least a Solution A which is a buffer solution having an electric conductivity of not less than 30 ms/cm; and a Solution B having an electric conductivity of not more than 6.5 ms/cm, the Solution B being a particle suspension which suspends particles which are insoluble carrier particles carrying a substance for capturing the analyte.