The present invention provides a method for diagnosing schizophrenia, and a schizophrenia diagnostic reagent or device for use in the method. The present invention further provides a therapeutic or ameliorating agent for schizophrenia, which is effective for the treatment or amelioration of schizophrenia. The therapeutic or ameliorating agent for schizophrenia contains a carbonyl scavenger or a carbonyl-modified protein formation inhibitor as an active ingredient. The method for diagnosing schizophrenia according to the present invention includes measuring at least one parameter in a subject, the parameter being selected from the group consisting of: (1) a genetic abnormality of glyoxalase I gene; (2) the expression level or activity of glyoxalase I in a biological sample; (3) the amount of a carbonyl compound or a carbonyl-modified protein that is a protein modified with the carbonyl compound; and (4) the amount of pyridoxal in a biological sample.