When a facsimile device detects an audio signal via a CNG signal detection unit while detecting an OFF signal in a CNG signal during a telephone conversation, the facsimile device does not detect the audio signal as an OFF signal if the level of the detected audio signal is greater than a threshold and the frequency of the audio signal is within a predetermined audio frequency range. Accordingly, the facsimile device can reliably prevent a false detection of a noise that has a similar signal component to a CNG signal as a CNG signal. It is possible to prevent the facsimile machine from being automatically shifted to a facsimile receiving mode by the false detection of the noise.