A method for detecting an analyte in a sample, the method comprising the steps of: forming on each of carrier particles a complex containing a first capture substance capable of binding to an analyte, one molecule of the analyte, a second capture substance capable of binding to the analyte, and a catalyst; immobilizing a reaction product on each of the carrier particles by reacting the catalyst in the complex with a substrate; and detecting the analyte by: - detecting the carrier particles on each of which the reaction product is immobilized by measuring optical information of the carrier particles and comparing a measured value of the optical information with a predetermined threshold value, wherein carrier particles in which the measured value is equal to or greater than the predetermined threshold are detected as carrier particles to each of which the reaction product is bound, - counting the carrier particles to each of which the reaction product is bound, - counting a total number of carrier particles subjected to detection, and - quantifying the analyte based on a result of counting the carrier particles to each of which the reaction product is bound and on a result of counting the total number, wherein one molecule of the analyte is captured per one carrier particle in the formation step; the reaction product is immobilized on the carrier particles on each of which the catalyst that has produced the reaction product is immobilized, but is not substantially immobilized on another carrier particle in the immobilization step; the formation step comprises a step of mixing the sample and a reagent containing a plurality of the carrier particles; and each carrier particle is not compartmentalized in the detection step and in the immobilization step. forming on each of carrier particles a complex containing a first capture substance capable of binding to an analyte, one molecule of the analyte, a second capture substance capable of binding to the analyte, and a catalyst; immobilizing a reaction product on each of the carrier particles by reacting the catalyst in the complex with a substrate; and detecting the analyte by: - detecting the carrier particles on each of which the reaction product is immobilized by measuring optical information of the carrier particles and comparing a measured value of the optical information with a predetermined threshold value, wherein carrier particles in which the measured value is equal to or greater than the predetermined threshold are detected as carrier particles to each of which the reaction product is bound, - counting the carrier particles to each of which the reaction product is bound, - counting a total number of carrier particles subjected to detection, and - quantifying the analyte based on a result of counting the carrier particles to each of which the reaction product is bound and on a result of counting the total number, - detecting the carrier particles on each of which the reaction product is immobilized by measuring optical information of the carrier particles and comparing a measured value of the optical information with a predetermined threshold value, wherein carrier particles in which the measured value is equal to or greater than the predetermined threshold are detected as carrier particles to each of which the reaction product is bound, - counting the carrier particles to each of which the reaction product is bound, - counting a total number of carrier particles subjected to detection, and - quantifying the analyte based on a result of counting the carrier particles to each of which the reaction product is bound and on a result of counting the total number, wherein one molecule of the analyte is captured per one carrier particle in the formation step; the reaction product is immobilized on the carrier particles on each of which the catalyst that has produced the reaction product is immobilized, but is not substantially immobilized on another carrier particle in the immobilization step; the formation step comprises a step of mixing the sample and a reagent containing a plurality of the carrier particles; and each carrier particle is not compartmentalized in the detection step and in the immobilization step.