Disclosed is a measurement method which can largely reduce the time required for the detection of a physiologically active substance derived from an organism (e.g., an endotoxin, β-D-glucan) or the determination of the concentration of the physiologically active substance by utilizing the reaction between the physiologically active substance and LAL. Also disclosed is a measurement apparatus utilizing the measurement method. An incident light from a light source is focused onto a sample and delivered to the sample to cause the bombardment with coagulin which is a final product of a protease cascade (i.e., a coagulin monomer) and an extremely fine aggregate which is produced by the aggregation of the coagulin monomers (i.e. a coagulin aggregate), thereby generating a scattered light. The scattered light is detected by a light-receiving element. The concentration of the endotoxin can be determined based on the initial rate of increase in the scattered light detected.