An electrical signal to be frequency-modulated is branched into first and second signals. An FM laser 102 oscillates light having a wavelength .lambda.1 in a steady state condition, and outputs an optical signal obtained by converting an amplitude change in the inputted first signal into an optical intensity change and an optical frequency change. An optical detecting portion 106, to which the optical signal from the FM laser 102 and light having a wavelength .lambda.0 outputted from a local light source 104 are inputted, converts an optical intensity modulation component of the inputted optical signal as a current amplitude change by operating square-law detection characteristics, and generates a beat signal at a frequency corresponding to a difference in wavelength .DELTA..lambda. between the two inputted optical signals. A discrimination portion 110 discriminates using a threshold value generated on the basis of the second signal and converts the beat signal into the pulse signal. A high frequency component of the pulsed beat signal is removed by an LPF 311. Consequently, it is possible to generate an ideal wide band FM modulation signal by removing an amplitude variation and an average value variation from the beat signal generated by using an optical frequency modulating operation of a laser diode and heterodyne detection.