PROBLEM TO BE SOLVED: To provide a bispecific antibody that is structurally stable, and can show alone a sufficient effect without the co-administration of activated lymphocyte (T-LAK).SOLUTION: A humanized highly functional bispecific antibody includes humanized variable regions of the heavy chain (5H) and the light chain (5L) of an anti-human EGF receptor 1 antibody 528, and humanized variable regions (OH) of the heavy chain and humanized variable regions (OL) of the light chain of the anti CD antibody OKT3 and has one of the following structures: any one of the single-chain polypeptides of (OH5L)-(a peptide linker)-(5HOL), (OH5H)-(a peptide linker)-(5LOL), or (5L5H)-(a peptide linker)-(OHOL) is bonded to two Fc regions of a human antibody via each hinge region.