In a measuring apparatus for measuring a target substance in a sample cell via a gelation reaction, a sample cell houses a specimen containing the substance to be measured, and a solution containing a gelating reagent is irradiated with a laser beam. The solution in the sample cell is stirred to generate minute and uniform gel particles, which are caused to pass through the laser beam. Scattered light from the gel particles generated in the sample cell is detected by a photodiode array, and the scattered-light intensity of the generated gel particles or the diameter and the number thereof is measured on time series by a computer on the basis of a scattered-light detection output of the photodiode array.