To provide an optical transmitter which can generate a modulated optical signal following a RZ pattern by only one optical modulator, without requiring a wide-band optical modulator or a driver circuit.

An inputted data signal, following a NRZ pattern inputted from an input terminal 10, is converted by a signal conversion section 11 into a binary output signal, which makes a level transition from '0' level to '1' level or vice versa, each time it transits from '0' level to '1' level, and when '1' level continues extended to two units period. Thereafter, the binary signal is inputted into a low-pass filter 12 to be band-limited. By driving a Mach- Zehnder type optical converter 16, based on an output signal of the low-pass filter 12 via a driver circuit 13 such a modulated optical signal 17, is outputted whose light intensity becomes a maximum, when the output signal of the low- pass filter 12 is at '0' level or at '1' level and becomes a minimum, when the output signal of the low-pass filter 12 is in a process of gradual transition between '0' level and '1' level.