A fragment antibody comprising a complex of: (a) a peptide (VH -SARAH); and (b) a peptide (VL-SARAH), characterized in that in the VH -SARAH, N-terminus of a SARAH domain is linked to a C-terminus of a heavy chain domain (VH region) of an antibody, the VH region has a site sufficient for recognizing a specific antigen and having a specific affinity binding property to the antigen, an amino acid residue of antibody residue 112 according to Chothia numbering scheme in the VH region is mutated to cysteine, and the SARAH domain is constituted of a short helix on the N-terminal side and a long helix on the C-terminal side; in the VL-SARAH, N-terminus of a SARAH domain is linked to a C-terminus of a light chain domain (VL region) of an antibody, an amino acid residue at position 13 from the C-terminus in the SARAH domain is mutated to cysteine, and the SARAH domain is constituted of a short helix on the N-terminal side and a long helix on the C-terminal side, the VH -SARAH and the VL-SARAH are linked by a disulfide bond between the two cysteines, and the long helix of the SARAH domain in the VH -SARAH has properties of forming antiparallel coiled coils between the long helix of the SARAH domain in the VL-SARAH. (a) a peptide (VH -SARAH); and (b) a peptide (VL-SARAH), characterized in that in the VH -SARAH, N-terminus of a SARAH domain is linked to a C-terminus of a heavy chain domain (VH region) of an antibody, the VH region has a site sufficient for recognizing a specific antigen and having a specific affinity binding property to the antigen, an amino acid residue of antibody residue 112 according to Chothia numbering scheme in the VH region is mutated to cysteine, and the SARAH domain is constituted of a short helix on the N-terminal side and a long helix on the C-terminal side; in the VL-SARAH, N-terminus of a SARAH domain is linked to a C-terminus of a light chain domain (VL region) of an antibody, an amino acid residue at position 13 from the C-terminus in the SARAH domain is mutated to cysteine, and the SARAH domain is constituted of a short helix on the N-terminal side and a long helix on the C-terminal side, the VH -SARAH and the VL-SARAH are linked by a disulfide bond between the two cysteines, and the long helix of the SARAH domain in the VH -SARAH has properties of forming antiparallel coiled coils between the long helix of the SARAH domain in the VL-SARAH. in the VH -SARAH, N-terminus of a SARAH domain is linked to a C-terminus of a heavy chain domain (VH region) of an antibody, the VH region has a site sufficient for recognizing a specific antigen and having a specific affinity binding property to the antigen, an amino acid residue of antibody residue 112 according to Chothia numbering scheme in the VH region is mutated to cysteine, and the SARAH domain is constituted of a short helix on the N-terminal side and a long helix on the C-terminal side; in the VL-SARAH, N-terminus of a SARAH domain is linked to a C-terminus of a light chain domain (VL region) of an antibody, an amino acid residue at position 13 from the C-terminus in the SARAH domain is mutated to cysteine, and the SARAH domain is constituted of a short helix on the N-terminal side and a long helix on the C-terminal side, the VH -SARAH and the VL-SARAH are linked by a disulfide bond between the two cysteines, and the long helix of the SARAH domain in the VH -SARAH has properties of forming antiparallel coiled coils between the long helix of the SARAH domain in the VL-SARAH.