Characteristics of a micro bubble generator using a strong swirl jet were studied experimentally and numerically. Experiments were conducted by changing the nozzle diameter, the inflow pressure and the gas flow rate, systematically. For measurements of the bubble distribution function of PDF, a simple method using a light transmission was proposed. Principle of the method was based on changes of the light transmission due to the bubble rising to the surface of water. The bubble distribution measured by the method agreed well with that by a usual backlight method. In this study four non-dimensional parameters governing the micro bubble generation were adopted, that is, Weber number for the average diameter of bubbles, Reynolds number, the ratio of flow rate and the velocity ratio for the swirl jet. It was found that the relationship between Weber number and Reynolds number could be arranged with each velocity ratio as a parameter. An approximate experimental expression for the average diameter showed that the micro bubble diameter strongly depended on the axial flow velocity and the rotational speed of the swirl jet.