Provided are a medical instrument capable of operating the entire treatment target site with a single irradiation by being provided with an irradiation range that can cover the entire treatment target site, and a light-ray treatment device. The medical instrument 30 is inserted into a living body and performs light irradiation on the treatment target site to treat the site. The medical instrument 30 comprises a lengthy medical instrument body 41 having at least one of lumens 42 and 43, and in the medical instrument body 41, lengthy light diffusers 44 and 45, and a lengthy reflector 46 having a reflection surface for reflecting light, each being inserted so as to be extended along the longitudinal direction of the medical instrument body 41. The medical instrument body 41 comprises, at least on a portion of the outer wall thereof, a transparent outer wall portion 41o that is optically transparent and extended along the longitudinal direction. The light diffusers 44 and 45 comprise a irradiation section 44 for irradiating light on the treatment target site and a light receiving section 45 for receiving light from the treatment target site, the light diffusers 44 and 45 being arranged so as to face the reflection surface and the transparent outer wall portion in a position between the reflection surface and the transparent outer wall portion.
