Provided is a method for using low molecular weight compounds found in the body as a biomarker for diagnosing depression. Specifically, more than one compound selected from a group comprising the following are used: ADP-ribose, ATP, ADP, AMP, serotonin, tryptophan, kynurenine, SDMA (symmetrical dimethylarginine), threonine, glyceric acid, serine, N-acetylaspartic acid, glutamic acid, trigonelline, creatine, 2-methylserine, sphingosine, homovanillic acid, piperidine, sulfoxidized methionine, pipecolic acid, sphinganine, gamma-butyrobetaine, guanidinoacetic acid, isobutyric acid, creatinine, sarcosine, 3-methylbutyric acid, nicotinamide, betaine, ornithine, carnitine, ethanolamine, phosphoethanolamine, taurine, hypotaurine, aspartic acid, methionine, and tyrosine.