Human adult hepatocytes are transplanted into an immunodeficient hepatopathy mouse and then human growth hormone is administered to the mouse to thereby elevate twice or more the replacement ratio by the human adult hepatocytes having been transplanted. Further, human growth hormone is administered to an immunodeficient hepatopathy mouse carrying human young hepatocytes transplanted thereinto so as to improve fatty liver of the mouse in which about 70% or more of the hepatocytes have been replaced by the human hepatocytes.