Low molecule agonist antibodies

The invention relates to a modified antibody which contains two or more H chain V regions and two or more L chain V regions of monoclonal antibody and can transduce a signal into cells by crosslinking TPO receptor to thereby exert TPO agonist action. The modified antibody can be used as a TPO signal transduction agonist and, therefore, useful as a preventive and/or remedy for various diseases such as platelet-reduction-related blood diseases, thrombopenia following chemotherapy for cancer or leukemia, etc.