A composition for diagnosing or detecting target cells through fluorescence imaging, comprising, as an active ingredient: an antigen responsive antibody-fluorescent dye conjugate which allows imaging diagnosis of a cell having a target antigen on a surface of a cell membrane, wherein the fluorescent dye is covalently bound to the antibody using a side chain of a lysine or cysteine residue present in a constant region of a light chain or a heavy chain in the antibody, wherein the antibody includes one or more tryptophan residues or a mixture of two or more of tryptophan, tyrosine, histidine, methionine residues, said residue(s) being present at a distance within 1.5 nanometers from the location at which the fluorescent dye is conjugated, wherein the fluorescent dye is quenched by photo-induced electron transfer (PET) with said residue(s) selected from the group consisting of tryptophan, tyrosine, histidine, and methionine in the antibody, and is dequenched to emit fluorescence on the surface of the cell membrane as a distance between the fluorescent dye and the residue(s) selected from the group consisting of tryptophan, tyrosine, histidine, and methionine in the antibody increases due to a three-dimensional conformational change of the antibody that occurs in a case where the antibody binds to the antigen present on the surface of the cell membrane, wherein the fluorescent dye is selected from the group consisting of ATTO 655 (trade name), ATTO 700 (trade name), ATTO MB2 (trade name) and ATTO 680 (trade name), wherein the antigen responsive antibody is selected from the group consisting of Trastuzumab, Cetuximab, anti-VEGF, anti-Vimentin and anti-CD44,, wherein Trastuzumab is conjugated with anyone of ATTO 655, ATTO 700, ATTO MB2 and ATTO 680; and anyone of Cetuximab, anti-VEGF, anti-Vimentin and anti-CD44 is conjugated to ATTO 680. an antigen responsive antibody-fluorescent dye conjugate which allows imaging diagnosis of a cell having a target antigen on a surface of a cell membrane, wherein the fluorescent dye is covalently bound to the antibody using a side chain of a lysine or cysteine residue present in a constant region of a light chain or a heavy chain in the antibody, wherein the antibody includes one or more tryptophan residues or a mixture of two or more of tryptophan, tyrosine, histidine, methionine residues, said residue(s) being present at a distance within 1.5 nanometers from the location at which the fluorescent dye is conjugated, wherein the fluorescent dye is quenched by photo-induced electron transfer (PET) with said residue(s) selected from the group consisting of tryptophan, tyrosine, histidine, and methionine in the antibody, and is dequenched to emit fluorescence on the surface of the cell membrane as a distance between the fluorescent dye and the residue(s) selected from the group consisting of tryptophan, tyrosine, histidine, and methionine in the antibody increases due to a three-dimensional conformational change of the antibody that occurs in a case where the antibody binds to the antigen present on the surface of the cell membrane, wherein the fluorescent dye is selected from the group consisting of ATTO 655 (trade name), ATTO 700 (trade name), ATTO MB2 (trade name) and ATTO 680 (trade name), wherein the antigen responsive antibody is selected from the group consisting of Trastuzumab, Cetuximab, anti-VEGF, anti-Vimentin and anti-CD44,, wherein Trastuzumab is conjugated with anyone of ATTO 655, ATTO 700, ATTO MB2 and ATTO 680; and anyone of Cetuximab, anti-VEGF, anti-Vimentin and anti-CD44 is conjugated to ATTO 680. <EOS>