A method of detecting a target substance, containing the steps of: incorporating labeling particles into a test liquid containing an analyte; heating the test liquid; irradiating the test liquid with excitation light, and detecting the target substance contained in the test liquid depending on a state of light emission of the labeling particles; wherein an aggregation state of the labeling particles is changed by the heating step; and wherein the labeling particles have a thermoresponsive polymer on a surface of a composite particle containing a magnetic material and a fluorescent material, and further have a biomolecule having properties of binding with the target substance. [Selected Drawing] Fig. 1