Disclosed is a return-to-zero (RZ) optical transmission device. A light source outputs a carrier wave. A precoder encodes an inputted non-return-to-zero (NRZ) electrical signal. A delay element delays the encoded signal. A Mach-Zehnder interferometer modulator has two electrodes. The Mach-Zehnder interferometer modulator configured to modulate phase and intensity of the carrier wave using respective output signals of the precoder the delay element applied, and output an RZ optical signal.