To have a remaining AM signal element eliminated by a small-sized structure and easy control.

This device is an optical heterodyne type FM modulator which, after generating an optical FM modulation signal by having an input signal drive a laser diode and multiplexing an optical local signal with an optical FM modulation signal, performs optical heterodyne detection, outputs the FM modulation signal that modulates a frequency of a signal having a difference frequency between both multiplexed signals. In this case, after a signal 1(t) and a signal-I (t) of a reversed phase to be inputted to an FM modulation laser diode 12 are generated, the optical FM modulation signal to be outputted from the FM modulation laser diode 12 is amplitude controlled by a refractive index changed-type light external modulator (an LN modulator) 22 so that an optical output is kept constant by the reversed phase signal, the amplitude controlled optical FM modulation signal and the optical local signal are multiplexed, then the optical heterodyne detection is performed to output the FM modulation signal.