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 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 that has a long tube and a long outer wall of the distal end portion, the wall being provided with electrodes exposed on the outer surface of the tube, and long light-diffusers disposed inside the outer wall of the distal end portion. The distal end portion includes a light transmission equalizer for equalizing light transmission along the length direction of the outer wall of the distal end portion.
