To clear the generation processes of the micro-bubbles in swirling flow-type micro-bubble-generators, we study the behavior of the air column, that rotates spirally about the axis of the micro-bubble-generator and the shift pattern from the air column to the micro-bubbles through the orifice plate for two types of micro-bubble-generators, by means of a high speed video camera with 250,000fps in framing rate. It is found that the break-down of the air column takes place around the orifice plate and the corresponding generation of the micro-bubbles.