Biomarker for diagnosing depression and use of said biomarker

The present invention provides a clinically useful and objective biomarker with which it is possible to evaluate the severity of depression. The present invention is a biomarker for evaluating the severity of depression, wherein the biomarker comprises at least one selected from the group consisting of 4-aminobutyric acid (γ(gamma)-aminobutyric acid: GABA), arginine, argininosuccinic acid, isoleucine, indolecarboxaldehyde, potassium indoleacetate, carnitine, acetyl carnitine, ornithine, xanthurenic acid, kynurenic acid, kynurenine, citric acid, creatine, creatinine, glutamine, dimethylglycine, serotonin, taurine, trimethyloxamine (TMAO), tryptophan, norvaline, 3-hydroxybutyric acid, phenylalanine, proline, betaine, and lysine.