The present invention relates to a polypeptide comprising an antigen binding domain and a carrying moiety having an inhibiting domain that inhibits the antigen binding activity of the antigen binding domain, and having a longer half-life than that of the antigen binding domain existing alone, methods for producing and screening for the polypeptide, a pharmaceutical composition comprising the polypeptide, methods for producing and screening for a single-domain antibody whose antigen binding activity is inhibited by associating with particular VL, VH or VHH, and a fusion polypeptide library including a single-domain antibody whose antigen binding activity is inhibited by associating with particular VL, VH or VHH.