To select and switch a system without hit when asynchronous serial signals are inputted from duplexed serial transmission lines.

The output signals of the serial transmission lines S0 and S1 are converted to parallel signals by serial-parallel conversion parts SP1 and SP2 beforehand and one of the parallel signals is selected by a parallel signal selection part 5. In the case of switching the system, deviation occurs in the output timing of the parallel signals, however, a parallel signal holding control part 9 generates control signals and the parallel signals are outputted from a signal holding part 6 while unrequired signals are obstructed.