In this study, noise sources of a reducer which is combined a spiral bevel gear and a planocentric gear are identified by experimental method and the noise was reduced by the structure modification of the reducer. In order to identify the noise sources, noise and vibration signals were measured by microphone and accelerometer and these signals were analyzed with waterfall plot. In addition, the frequency response functions were obtained to prove the noise and vibration sources. It was found that the resonance was generated by the gear mesh frequencies and natural frequency of the reducer. The noise of the reducer could be reduced by structure modification.