To increase a transmission capacity of a digital signal without changing a band width of a transmission channel.

A modulator has a controller(CTRL) that assigns a digital signal to be sent to some carrier frequencies in a transmission channel. In the most simple case, on the occurrence of a bit sequence 01, a 2nd carrier frequency (f2) is sent without transmission of the 1st carrier frequency (f1), and on the occurrence of a bit sequence 11, both the carrier frequencies (f1, f2) are sent simultaneously. Thus, two bit sequence sets are assigned to the two carrier frequencies of the transmission channel. On the occurrence of the bit sequence 11, it is not required to send the carrier frequency twice continuously, and thus the transmission capacity is increased.