An analogue audio signal input to a transmission unit 1 is band limited into a frequency band lower than that of a main signal pass with a low-pass filter 21 and then converted into a digital value with a sampling clock lower than the main signal pass by an analogue/digital converter 22. All bits of the converted digital signal are inverted by an inverter 23. The digital signal input to a reception unit 3 via a transmission channel 100 is inverted by inverter 42 and is digital/analogue converted by digital/analogue converter 43 with the sampling clock lower than the main signal pass. The resultant signal is band limited into the low frequency band through a low-pass filter 44 and then output to a level comparator 45. A low-pass filter 46 band limits the signal for the main signal pass into the low frequency bang. The level comparator 45 compares the output level of the low-pass filter 44 with that of the low-pass filter 46.