Neuroimaging technologies have seen enormous progress during the past two decades, and are now able to provide detailed illustrations of brain structure and activity. As a result, our understanding of the pathophysiology of mental illnesses such as schizophrenia has expanded, and potential clinical applications are becoming apparent. Unlike most physical illnesses, mental illnesses often lack objective markers, and this is one of the reasons why some patients and their family members experience difficulty in understanding symptoms and disabilities. In some cases, this leads to delays in patients seeking treatment, discontinuation of treatment, and relapse. If biological markers can be utilized as aids in differential diagnosis, selection of treatment options, and assessment of severity and treatment response in clinical settings, they may lead to improved treatment quality and clinical outcomes. Near‒infrared spectroscopy（NIRS）is a functional neuroimaging technique that may be a candidate for clinical application in psychiatry. Advantages of NIRS include：1）its non‒invasive nature, with no harmful impact resulting from repeated measurement, even in infants；2）high temporal resolution（＞10 Hz）；3）compact size, enabling easy movement；and 4）no required restraint position：people undergoing testing may take a natural position, such as sitting or standing, and may converse and perform manual tasks during measurement. Previous NIRS studies have revealed different characteristic waveform patterns during verbal fluency tasks among patients with major depression, bipolar disorder, and schizophrenia, which led to government approval in 2009 as an“advanced medical technology to aid in the differential diagnosis of depressive symptoms.”This was the first approval in the field of psychiatry in Japan. Future NIRS studies will focus more on clinical applications, and multi‒site studies are needed to assess the impact of non‒illness factors such as age, sex, and head size, in addition to factors related to illness, including differences in sub‒type diagnosis, severity of symptoms, and treatment. It is hoped that findings will support the development of new biomarkers that will facilitate differential diagnosis, better understanding of present conditions and prognosis, and decisions regarding treatment options.