A method for detecting an analyte, the method comprising: a first step of supplying a specimen to a detection device having a first ligand, the first ligand being immobilized on a substrate and capable of specifically binding to the analyte, the specimen being supplied onto the substrate of the detection device, and then causing the analyte included in the specimen to bind to the first ligand; wherein the substrate has a support; a metal member formed on the support; a self-assembled monolayer (SAM) formed on the metal member; and a hydrophilic polymer layer formed on the SAM and the first ligand is immobilized to the hydrophilic pol-ymer layer; wherein the detection device is a plasmon excitation sensor; and wherein the hydrophilic polymer layer is compoased of carboxymethyl dextran; a second step of supplying a second ligand onto the substrate, the second ligand being labeled with a marker and capable of specifically binding to the analyte, and then causing the second ligand to bind to the analyte bound to the first ligand; wherein in the second step carboxmethyl dextran is supplied onto the substrate; the carboxymethyl dextran having a molecular weight of from 40.000 to 750.000, a third step of measuring the second ligand bound to the analyte; and wherein the specimen has been diluted with a solvent selected from the group consisting of phosphate buffered saline, Tris buffered saline and HEPES buffered saline. a first step of supplying a specimen to a detection device having a first ligand, the first ligand being immobilized on a substrate and capable of specifically binding to the analyte, the specimen being supplied onto the substrate of the detection device, and then causing the analyte included in the specimen to bind to the first ligand; wherein the substrate has a support; a metal member formed on the support; a self-assembled monolayer (SAM) formed on the metal member; and a hydrophilic polymer layer formed on the SAM and the first ligand is immobilized to the hydrophilic pol-ymer layer; wherein the detection device is a plasmon excitation sensor; and wherein the hydrophilic polymer layer is compoased of carboxymethyl dextran; wherein the substrate has a support; a metal member formed on the support; a self-assembled monolayer (SAM) formed on the metal member; and a hydrophilic polymer layer formed on the SAM and the first ligand is immobilized to the hydrophilic pol-ymer layer; wherein the detection device is a plasmon excitation sensor; and wherein the hydrophilic polymer layer is compoased of carboxymethyl dextran; a second step of supplying a second ligand onto the substrate, the second ligand being labeled with a marker and capable of specifically binding to the analyte, and then causing the second ligand to bind to the analyte bound to the first ligand; wherein in the second step carboxmethyl dextran is supplied onto the substrate; the carboxymethyl dextran having a molecular weight of from 40.000 to 750.000, a third step of measuring the second ligand bound to the analyte; and wherein the specimen has been diluted with a solvent selected from the group consisting of phosphate buffered saline, Tris buffered saline and HEPES buffered saline. <EOS>