A premixed-combustion type boiler has a strong tendency to generate extremely loud combustion noise. In order to use the boiler in practice, it is necessary to minimize such noise. Therefore, in this paper, a noise reduction method is proposed, in which an extra exciting frequency signal may cause the self-sustained frequency intrinsic in the system altered with a synchronization. The boiler system is replaced with a model apparatus which can generate a self-sustained oscillation noise as is often the case in the Rijke tube. In the model apparatus, basic feature of the proposed method and its effectiveness are made clear. By enforcing an exciting frequency signal from outer system, self-sustained frequency can be transferred to a frequency range where system cannot produce oscillation noise. Thus it became evident that the sound level of the total system can be lowered to a large extent and the proposed method is effective.