A method of producing a labeled antibody, including the steps of: a) allowing silica nanoparticles containing a functional molecule and having a thiol group on a surface thereof, and a linker molecule containing a maleimido group and an amino group, to coexist in a solvent to form a thioether bond between the thiol group and the maleimido group, thereby obtaining functional molecule-containing silica nanoparticles on which the linker molecule is bonded; and b) allowing the functional molecule-containing silica nanoparticles on which the linker molecule is bonded, carbodiimide and an antibody to coexist in an aqueous solvent to form an amide bond between the amino group of the linker molecule and a carboxyl group of the antibody.