Demand for inexpensive and wide band microphone for machine noise in factory and power station is increasing. However, frequency response is limited by diaphragm in conventional microphone. In this paper, a laser microphone based on self-coupling effect of the semiconductor laser is proposed. The laser microphone has a wide and flat frequency response. The features of the laser microphone do not need optical adjustment with simple optical system and not need post-processing by digital signal processing. The laser microphone can detect a sound wave and a human voice. This signal voltage is proportional to sound pressure and flat in frequency response in audible sound. This microphone has a omni-directivity in vertical direction.