It is intended to provide highly stable variants of human antibody IgG2 and IgG3 subclasses. The present invention provides an IgG heavy chain comprising the constant region of a human IgG2 heavy chain having at least a substitution of Y for F at the 300th position, L for V at the 309th position, or A for T at the 339th position designated by the EU index of Kabat et al. and an IgG heavy chain comprising the constant region of a human IgG3 heavy chain having at least a substitution of K for N at the 392nd position or V for M at the 397th position designated by the EU index of Kabat et al. The present invention also provides monoclonal antibodies comprising these heavy chains.