To switch lines so as not to cause data omission by making the phases and frequencies of modulation data coincide with each other about data transmis sion which transmits the same modulation data through different lines and switches the lines to make it possible to receive the modulation data when a line fails.

This device is provided with a delay correcting part 1 which inputs modulation data to a line switching device through two lines with the modulation data controlled to the same phase and a frequency correcting part 2 making the carrier frequencies of the modulation data coincide, the part 1 has a correlator 11 calculating the correlation value of the modulation data through the two lines and a variable delay circuit 12 controlling delay quantity so that the correlation quantity can be maximum, and the part 2 has a frequency comparing part 10, a voltage controlled oscillator 4 controlling an oscillation frequency in accordance with frequency difference, a reference oscillator 3, 1st and 2nd multipliers 6 and 7 and 1st and 2nd bandpass filters 8 and 9.