The present invention relates to a lake water purification apparatus using microbubbles, and more particularly, to a lake water purification apparatus which removes, through coagulation and floatation, a variety of contaminants such as floating materials, organic contaminants, nutrient salts, waterweeds, and the like present under water and in bottom sediment from the contaminated lake by using a microbubble generator, an auto fence, and an elution line to restore and improve the water quality of lakes and marshes. The lake water purification apparatus of the present invention includes a fence arranged on the surface of the water to define a predetermined zone; a microbubble generator (hereinafter, MBP) for generating bubble water containing microbubbles and a coagulant from a predetermined depth of water to coagulate and float contaminants into the fence; a collection means arranged in the fence to move, to one side, sludge in which the contaminant is coagulated and floated by the bubble water; a sludge reservoir arranged at one end of the fence to store the sludge collected by the collection means; a first water level sensor for sensing the level of the sludge collected in the sludge reservoir; a control unit for controlling the operation of a sludge delivery pump in accordance with the signal output from the first water level sensor; and a sludge delivery pump arranged in the vicinity of the sludge reservoir to deliver the sludge to a ground reservoir. The bubble water is mixed and reacted with the lake water while maintaining a predetermined flow, and coagulated into flocks such that microbubbles are adsorbed to the flocks, floated on the surface of water, and collected in the fence. The microbubbles are then collected into the sludge reservoir by the collection means.