A new type of optical-microwave converter utilizing a photomixer and a high-Tc Josephson junction detector has been developed. The frequency difference of two laser beams has been observed in the characteristic of the high-Tc Josephson junction detector. The measured response of the detector for the microwave generated by photomixing two lasers suggests that the system can work as a highly sensitive wavelength meter with accuracy of ±3.5 pm. The system operation up to 20 GHz was also demonstrated by changing the wavelength of two laser beams.