A method for detecting a test substance in a sample by an immune complex transfer method, comprising the steps of: bringing an antibody solution comprising a labeled antibody capable of binding to the test substance into contact with a first solid phase and/or a second solid phase used in the immune complex transfer method; separating the antibody solution from the first solid phase and/or the second solid phase to obtain the antibody reagent; mixing the antibody reagent, a sample comprising the test substance, a reagent comprising a capture substance capable of binding to the test substance and wherein the capture substance comprises a first binding partner capable of binding to a first binding substance and a second binding partner capable of binding to a second binding substance, and a first solid phase to which the first binding substance is immobilized and immobilizing an immune complex comprising the labeled antibody comprised in the antibody reagent, the test substance, and the capture substance on the first solid phase, removing a free component not contained in the immune complex from the mixture obtained in the immobilization step, releasing the immune complex from the first solid phase, transferring the released immune complex onto a second solid phase different from the first solid phase, wherein the second binding substance is immobilized on said second solid phase, and detecting a signal based on the labeled antibody comprised in the immune complex on the second solid phase and detecting the test substance based on the signal, wherein the capture substance binds to a site different from the site to which the labeled antibody in the antibody reagent binds the test substance. bringing an antibody solution comprising a labeled antibody capable of binding to the test substance into contact with a first solid phase and/or a second solid phase used in the immune complex transfer method; separating the antibody solution from the first solid phase and/or the second solid phase to obtain the antibody reagent; mixing the antibody reagent, a sample comprising the test substance, a reagent comprising a capture substance capable of binding to the test substance and wherein the capture substance comprises a first binding partner capable of binding to a first binding substance and a second binding partner capable of binding to a second binding substance, and a first solid phase to which the first binding substance is immobilized and immobilizing an immune complex comprising the labeled antibody comprised in the antibody reagent, the test substance, and the capture substance on the first solid phase, removing a free component not contained in the immune complex from the mixture obtained in the immobilization step, releasing the immune complex from the first solid phase, transferring the released immune complex onto a second solid phase different from the first solid phase, wherein the second binding substance is immobilized on said second solid phase, and detecting a signal based on the labeled antibody comprised in the immune complex on the second solid phase and detecting the test substance based on the signal, wherein the capture substance binds to a site different from the site to which the labeled antibody in the antibody reagent binds the test substance. <EOS>