A method and an apparatus are provided for measuring trace ingredients in water, which make it possible to perform detection and quantitative determination of silica, phosphorus or arsenic in a sample solution. The method includes adding a molybdate ion to a sample solution under an acidic condition, then adding a fluorescent counter cationic pigment thereto, and measuring the time required for the fluorescence emitted by the resultant solution to be attenuated to a prescribed intensity. The measuring apparatus includes an excitation light source which irradiates excitation light to the above-modified solution; a fluorescence detector which detects the fluorescence emitted by the solution; means for measuring the time required for the fluorescence detected by the fluorescence detector to be attenuated to a prescribed intensity; and control means for determining the concentration of silica, phosphorus or arsenic in the sample solution from the time measurement.