Provided are a medical instrument capable of operating the entire treatment target site with a single irradiation. The medical instrument is inserted into a living body and performs light irradiation on the treatment target site. The medical instrument includes a lengthy medical instrument body having at least one of lumens, and in the medical instrument body, lengthy light diffusers, and a lengthy reflector having a reflection surface for reflecting light, each being inserted to be extended along the longitudinal direction. The medical instrument body includes a transparent outer wall portion that is optically transparent and extended along the longitudinal direction. The light diffusers includes an irradiation section for irradiating light on the treatment target site and a light receiving section for receiving light from the treatment target site, the light diffusers being arranged to face the reflection surface and the transparent outer wall portion.
