Transmission/reception device and transmission/reception method

A transmission/reception device comprising:

• a photo-electric converter to receive an optical signal based on a plurality of first optical signals having frequency bands different from each other, convert the optical signal into an electric signal, and output the converted electric signal as a plurality of first electric signals;
• a guard band remover to receive the plurality of the first electric signals output from the photo-electric converter, change the frequency bands of some or all of a plurality of second electric signals that are electric signals corresponding to the respective frequency bands of the plurality of first optical signals based on the plurality of first electric signals to narrow an interval between the frequency bands of the two second electric signals having frequency bands adjacent to each other, and output electric signals each of which corresponds to each of the plurality of second electric signals after the interval has been narrowed, as a plurality of third electric signals;
• an electric signal combiner to receive the plurality of third electric signals output from the guard band remover, combine the plurality of third electric signals, and output a combined electric signal as a fourth electric signal;
• and
• an electric-photo converter to receive the fourth electric signal output from the electric signal combiner, convert the fourth electric signal into an optical signal, and output the converted optical signal as a second optical signal.