It is an object of the present invention to provide an immunoassay method which is capable of simultaneous quantification of a plurality of test substances under a same analysis condition, through adjustment of the measurement sensitivity/range by simply varying the size of particles for use as labels, without changing the spectra of these particles. The present invention provides an immunoassay method which comprises: simultaneously or step-by step developing respective of the plurality of the test substances and respective of labeling substance particles labeled with first binding substance against the respective of the plurality of the test substances on an insoluble carrier; capturing the test substances and the labeling substance particles in reaction portions on the insoluble carrier that has been immobilized with the respective of second binding substances against the respective of the plurality of the test substances at different positions; and measuring optical characteristic of the labeling substance particles, so as to simultaneously detect the plurality of the test substances, wherein the plurality of the test substances having different detection concentration ranges are simultaneously detected with use of the labeling substance particles which are different in the particle size corresponding to the test substances.