Optical transmission being performed by frequency modulating frequency-division-multiplexed multi-channel signals as a single unit, distortion that is caused by a ripple-shaped group delay deviation in a transmitter 2, a receiver 4 and an optical link 3 is reduced by providing a level adjuster 2a for adjusting the level of frequency-division-multiplexed multi-channel signals input to the FM modulator 2b, and by increasing the input level of multi-channel signals to the FM modulator 2b by level adjusting means 2a to enlarge the bandwidth of FM signal, in the case where the number of channels is small.