To prepare an antibody conjugate capable of binding to a radionuclide metal and usable for the in vivo diagnosis, treatment, etc., by binding an bifunctional coupling agent having an S-acylmercapto group capable of forming a complex compound with a radionuclide metal ion to an antibody.

This new antibody conjugate is represented by formula I [Ab is a residue of an antibody or its fragement; R1 is a 1-6C alkyl, a 6-8C aryl or a 7-9C alkaryl and any of which can be substituted with one or more hydroxyl, alkoxyl, carboxy or sulfonate groups; (n) is either 1 or 2; AA is an α- or a β-amino acid linked to each other with amide bonds; (i) is 2-6] or formula II. The conjugate is capable of binding to a radionuclide metal ion and useful for the in vivo diagnosis, treatment, etc. The antibody conjugate is prepared by using an antibody against a tumor associated antigen as an antibody, enzymically cleaving an Fc fragment from the antibody, providing an F(ab)'2 dimer, reducing the resultant dimer, affording an Fab' fragment and reacting the resultant fragment with a bifunctional coupling agent represented by formula I or II.