Abstract: An apparatus for measuring liquid-gas two-phase flow comprises a light source, an optical fiber with the end face disposed perpendicularly to the flow direction of the two-phase flow, and a photo detector which detects the reflected light. When a bubble approaches toward the end face of the fiber and the center thereof pass through it, the light reflected by the bubble is shifted by the Doppler effect and the light intensity reflected by the end face of the fiber changes corresponding to the difference of the refraction indices of the liquid and gas, so that the photo detector generates electric signal from light beat of these reflected light beams. And bubble velocity and its diameter can be simultaneously measured by high and low frequency components of this signal, respectively.