PURPOSE:To attain sure data transmission by returning a transmission band switching request signal to the transmission side to switch the transmission band when plural transmission bands are prepared and a setting value or over of the transmission characteristic is deteriorated at the reception side. CONSTITUTION:When the transmission characteristic is changed in a power line 9, since the error rate of a reception data is increased more than a setting value, a band switching request signal is generated, it is subject to spread spectrum modulation and the result is returned to the transmission side. A modulation signal fed from the power line 9 is extracted by a coupler 8, amplified (13) and fed to a data demodulation section 14 to apply inverse spread spectrum demodulation thereby extracting the band switching request signal and the transmission band is switched by a control section 15.
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