Disclosed is a communication system, having: a transmitter including: a first frequency converter converting a frequency of a provided signal to a first frequency band to generate a first signal and converting the frequency of the provided signal to a second frequency band to generate a second signal, the second frequency band having a given frequency difference against the first frequency band; and a combiner combining the first signal and the second signal to output a transmission signal, and a receiver including: a second frequency converter converting the transmission signal to a frequency-shifting signal shifted by the given frequency difference; a first multiplier multiplying the transmission signal and the frequency-shifting signal together to output a multiplied signal; a filter extracting a baseband signal from the multiplied signal; and a detector detecting significant information from the baseband signal.