To improve the efficiency of diving operation and to secure the safety of a diver.

In this suppressing method, an input signal is divided into minute time sections, it is judged whether each section includes a voice or not, the prescribed time range before and after a section discriminated as including a voice is judged as a voice signal section, while a signal level of a part discriminated as other than a voice signal section. And it is judged whether a voice is included or not using a cepstrum analysis result by examining existence of a signal exceeding a threshold value calculated conforming to a fixed regulation in a range of a pitch previously decided.