An optical wavelength converting apparatus comprises a semiconductor laser, which is capable of oscillating in a multiple longitudinal mode, and an external resonator for resonating a laser beam, which has been radiated out of the semiconductor laser and which serves as a fundamental wave. An optical wavelength converting device is located in an optical path of the fundamental wave, which has been resonated by the external resonator. A device feeds part of the fundamental wave, which has been resonated by the external resonator, back to the semiconductor laser. A wavelength-converted wave in a single longitudinal mode is thereby obtained, which wave has a high intensity and is stable.