To silence a handpiece noise over a wide frequency band in both of air conduction and bone conduction.

This silencing system of odontotherapy apparatus collects a handpiece rotating sound, which is a mental affliction for a patient under a treatment, in the oral cavity of the patient by a handpiece built-in microphone 12, and outputs a phase inversion sound from an air conduction speaker 42 to cancel it. This system allows a patient ear microphone 13 to collect the uncanceled sound (sound deviation), refers to the digital conversion value and the rotation frequency in operating the handpiece and/or racing it stored in a memory, extracts a difference value determined to pertain to the frequency band of the handpiece rotating sound, and corrects the sound deviation by a feedback control of adjusting the phase for canceling the difference value. When an operation determining means ALU determines the operation of the handpiece, the phase inversion sound, or the bone conduction sound, collected by a tooth press-fitted microphone 14 is output from the bone conduction speaker 3 to cancel it.