PURPOSE: To enable the simultaneous modulation transmission for analog and digital signal, by converting the analog signal into the pulse signal given by the position modulation based on its signal, superimposing the signal given by the amplitude modulation according to the digital signal on this signal, and by performing optical modulation for this superimposed signal.

CONSTITUTION: The pulse position modulator 1 which gives the position modulation based on the analog signal A to the analog signal A and provides the pulse generator 3 converting the analog signal into the pulse signal b, and the pulse amplitude modulator 2 provided with the pulse counter 4 which gives the amplitude based on the digital signal D to this converted pulse signal b, are provided. Further, the pulse signal b is superimposed on the digital signal D, the pulse signal d superimposing the analog signal A and the digital signal D on the output of the modulator 2, is outputted. This pulse signal d is modulated into photo signal and is fed to the photo transmitter 5 which delivers it to the optical fiber 6.