Water purification of lower layer in a lake of dam was investigated using micro bubble technology. A device for generating pure oxygen micro bubbles in the lower layer of the lake of dam was developed. Almost all micro bubbles formed by the micro bubble generator shrank and changed to micro-nano bubbles in tap water. The rising velocity of micro bubbles obeyed Stokes' law. The dissolved oxygen concentration of the lower layer in the wide area of the lake increased rapidly after providing pure oxygen micro bubbles. In addition, the turbidity increased temporarily by the supply of the bubbles, but the increasing area of the turbidity stayed only in the lower layer of the lake.