The MM-22 medical microtron at Korea Cancer Center Hospital has been running for radiotherapy since it was installed in 1986. The microtron is a very flexible radiation therapy device with excellent radiation field for photon or electron therapy. The microtron accelerates elections from an energy of minimun 5.3MeV to an energy of maximum 22.5MeV. The electrons are led from the microtron to the treatment head via a beam transport system and are used for radiotherapy. Present paper describes the system structures and operating characteristics of the MM-22 microtron and its therapy unit.