Near-infrared spectroscopy (NIRS) is a functional neuroimaging technique that has been increasingly employed in psychology and psychiatry. Because NIRS can detect only cerebral cortex reactivities with low spatial resolution and may suffer from contaminating signals from the skin and skull, its data should be interpreted as a global index of cerebral cortex reactivities. Within these limitations, the advantages of NIRS over fMRI such as complete non-invasiveness, small measurement apparatus, high time resolution, and natural examination setting, makes it the preferred method in studies of brain substrates of subjective feelings of sleepiness and fatigue, personality, conversation, and psychiatric disorders. Two-thirds of the original articles on NIRS applications in psychiatry have been published by Japanese researchers. NIRS examination of psychiatric disorders using a verbal fluency task of only 3 minutes demonstrated their characteristics of frontal lobe function: depression was characterized by smaller activation, bipolar depression by comparable but delayed activation, and schizophrenia by reduced activation followed by re-activation during the post-task period. These characteristics can also be identified in individual NIRS data using 2 automatically calculated parameters. Based on these results, NIRS application in psychiatry has been approved as one of the Advanced Medical Technologies in 2009 as an aid for differential diagnosis of depressive symptoms. A lack of clinical laboratory tests for diagnosis and treatment has been one of the major difficulties for reliable diagnosis, quantitative treatment assessment, and prevention of psychiatric disorders; NIRS may be the first step toward such clinical laboratory tests in psychiatry.