The present invention provides a medical device and a phototherapy system that include a light diffuser that transmits light to a site to be treated and electrodes for measuring electric potential and that allow phototherapy and measurement of electric potential using a single device.A medical device 30 is inserted into a living body and transmit light to a site to be treated to treat the site. The medical device includes a distal end portion 44t that has a long tube 41 and a long outer wall of the distal end portion, the wall being provided with electrodes 49T and 49R exposed on the outer surface of the tube 41, and long light-diffusers disposed inside the outer wall of the distal end portion. The distal end portion 44t includes a light transmission equalizer 49h for equalizing light transmission along the length direction of the outer wall of the distal end portion.
