To realize an efficient and economic total optical network by multiplexing plural comparatively low-rate optical signals, line-concentrating them, converting them into higher-rate electrical signals and newly converting the electrical signals into optical signals with a certain optical wavelength.

Optical signals λ11 to λ1i, λ21 to λ2j and λ31 to λ3k with a comparatively low bit rate at a tributary side are respectively converted into the electrical signals by opto-electric transducer parts 1, 3 and 5. The converted electrical signals are multiplexed by time division by TDM MUX 7, converted into the higher-rate electric signals and converted into the high-rate optical signal λ1 by an electro-optical converting part 8. Similarly, the optical signals λ41 to λ4a and λ51 to λ5b with the comparatively high bit rate are converted into the optical signals λ2 to λc and λd to λn. Thus, the optical signals at the tributary side are concentrated, converted in wavelength, converted into the optical signals λ1 to λn, multiplexed in wavelength by WDM MUX 9, converted into one optical signal, and transmitted.