The preparation of nanoscale fine bubbles in water is an innovative technology, but no precise method for simultaneously measuring the size and concentration of such bubbles had previously been developed. We have developed a method for simultaneously determining the size and concentration of fine bubbles in water by a light-scattering technique. Dynamic light scattering gives the diffusion constant and particle size of fine bubbles, whereas static light scattering provides their concentration or molar mass. Static light scattering also provides the radius of gyration of the bubbles, thereby providing a means for validating measurements of the sizes of the fine bubbles.