PURPOSE: To transfer the multiplex information of time division frequency, by multiplexing the time division multiplex information moreover in a frequency area, and transferring the digital signal as it is through an optical transmission line.

CONSTITUTION: Digital information in a memory 1 is read out by a control device or the like in a prescribed sequence and inputted to respective transmission parts 21∼2n. In the transmitting part 21, P-S converters 2101∼210m convert the input signal to serial ones and output the serial signals to an multiplexer MPX 211, the MPX 211 multiplexes these outputs with time division and outputs the multiplexed signal to a laser diode 212, and the diode 212 transmits the input signal together with the laser having the oscillation frequency f1 to an optical fiber 3. Since other transmission parts 22∼2n also transmit signals to the fiber 3, the time division multiplex information is moreover multiplexed at their frequency and transmitted as digital information. Receiving parts 41∼4n separate the multiplex digital information in time division and, after S-P conversion, the separated information is stored in a memory 5.