PURPOSE: To eliminate disturbance of identification of a peak of an object to be analyzed on chromatography as caused by the excessive existence of a fluorescent reagent by causing an ammonia salt to react with an excessive fluorescent reagent to be used in a fluorescence analysis.

CONSTITUTION: A quenching of an excessive fluorescent reagent is performed by causing an ammonia salt to react with the excessive fluorescent reagent to be used in a fluorescence analysis. The fluorescent reagent herein used is, for example, fluorescein isothiocyanate (FITC). The ammonia salt for the excessive reagent is used normally with the concentration exceeding 0.1mol. When the ammonia salt is added to the excessive fluorescent reagent in the sample to react, as mentioned above, it is preferable to heat the reagent at about 55°C normally in view of reactivity. Reaction time is normally sufficient, about 10 minute. By such a treatment, aminofluorescein produced from the excessive FITC does not affect analysis of a compound to be detected even when existing in the sample and the intensity of the fluorescence is half as much as FITC thereby eliminating the need for the removal thereof from the sample.