To provide a device which carries out the locking of the conjugate of a four-wave signal generated from a slave laser by introducing a laser beam generated by the master laser on the slave laser, and oscillates the light of the millimeter wave.

The tunable optical oscillator includes an electrical signal generator 11 to generate a predetermined electrical signal, a master laser 12 which is driven by the electrical signal and generates and outputs an optical signal having a first frequency, and a slave laser 13 which is driven by the electrical signal, generates an optical signal having a second frequency, and mixes the optical signal in which the optical signal outputted from the master laser 12 is inputted with the optical signal having the second frequency to be outputted. An ultra high-speed optical signal generation technology through a high speed modulation by this constitution is capable of generating optical signals over a broadband including tens of MHz to several THz. Thereby, a semiconductor laser or optical element such as an optical modulator requested in an ultra high-speed optical communication system is realized.