A method of producing a probe-bound carrier, which comprises: step 1 of mixing a carrier having tosyl groups and a probe; and step 2 of reducing the amount of tosyl groups on a surface of a carrier, wherein S2/S1A × 100% is not less than 140%, wherein S2 is an area that is occupied by one tosyl group on a surface of the carrier obtained in the step 2 and S1A is an area that is occupied by one tosyl group on a surface of the carrier used in the step 1 wherein the step 1 comprises: step 1a of mixing a carrier having tosyl groups with ammonium sulfate; and step 1b of mixing in a probe after the step 1a, wherein the step 2 is a step of mixing the carriers obtained in the step 1 with a blocking agent wherein the blocking agent is a substance having a functional group that is reactive to a tosyl group and the blocking agent is at least one substance selected from the group consisting of tris(hydroxymethyl)aminomethane, ethanolamine, casein, gelatin and a synthetic polymer, wherein the carrier is a magnetic particle, wherein the area S1A is not less than 20 Å 2 /tosyl group, wherein an area 51B that is occupied by one tosyl group on a surface of the carrier obtained in step 1 is not less than 10 Å 2 /tosyl group, wherein S2/S1B × 100% is not less than 110%, and wherein the area occupied by one tosyl group on the surface of a magnetic particle is determined by the following formula. step 1 of mixing a carrier having tosyl groups and a probe; and step 2 of reducing the amount of tosyl groups on a surface of a carrier, wherein S2/S1A × 100% is not less than 140%, wherein S2 is an area that is occupied by one tosyl group on a surface of the carrier obtained in the step 2 and S1A is an area that is occupied by one tosyl group on a surface of the carrier used in the step 1 wherein the step 1 comprises: step 1a of mixing a carrier having tosyl groups with ammonium sulfate; and step 1b of mixing in a probe after the step 1a, wherein the step 2 is a step of mixing the carriers obtained in the step 1 with a blocking agent wherein the blocking agent is a substance having a functional group that is reactive to a tosyl group and the blocking agent is at least one substance selected from the group consisting of tris(hydroxymethyl)aminomethane, ethanolamine, casein, gelatin and a synthetic polymer, wherein the carrier is a magnetic particle, wherein the area S1A is not less than 20 Å 2 /tosyl group, wherein an area 51B that is occupied by one tosyl group on a surface of the carrier obtained in step 1 is not less than 10 Å 2 /tosyl group, wherein S2/S1B × 100% is not less than 110%, and wherein the area occupied by one tosyl group on the surface of a magnetic particle is determined by the following formula. step 1a of mixing a carrier having tosyl groups with ammonium sulfate; and step 1b of mixing in a probe after the step 1a, wherein the step 2 is a step of mixing the carriers obtained in the step 1 with a blocking agent wherein the blocking agent is a substance having a functional group that is reactive to a tosyl group and the blocking agent is at least one substance selected from the group consisting of tris(hydroxymethyl)aminomethane, ethanolamine, casein, gelatin and a synthetic polymer, wherein the carrier is a magnetic particle, wherein the area S1A is not less than 20 Å 2 /tosyl group, wherein an area 51B that is occupied by one tosyl group on a surface of the carrier obtained in step 1 is not less than 10 Å 2 /tosyl group, wherein S2/S1B × 100% is not less than 110%, and wherein the area occupied by one tosyl group on the surface of a magnetic particle is determined by the following formula.