To maintain the signal transmission quality at a constant level even if the signal of one or multiple channels erase while optical signal is transmitted or signal is supplied to the idle channel or erasure channel in a device of a system which multiplex signal of multiple channels and transmit as an optical signal of constant average optical power.

An optical time division multiplex part 1 always monitors whether signal is normally supplied to n channels or not. When the signal of at least one channel erases and a duty ratio of optical signal reduces, the optical time division multiplex part 1 sends a channel information signal which shows an erasure channel of the signal to a transmission optical amplifier 2. The transmission optical amplifier 2 avoids increasing in power of each optical pulse by controlling optical output in dependence upon the channel information signal.