Disclosed herein is a sterilizer used at dental clinics to sterilize handpieces by the irradiation of ultraviolet rays. When inserting a contaminated handpiece into a sterilizer, an ultraviolet barrier is opened confirming the approach of the handpiece by a sensor. After the insertion of the handpiece, the ultraviolet barrier is closed, then ultraviolet lamps are turned on for a pre-determined time and then turned off, and the ultraviolet barrier is opened again. After the withdrawal of the handpiece, the ultraviolet barrier is closed. The handpiece is not contacted with the outside of the sterilizer, preventing the handpiece from being contaminated again by the sterilizer, and the leakage of the ultraviolet rays is prevented.