It has been an increasing concern that the depletion of Dissolved Oxygen (DO) concentration at the bottom layer of the northern-part of Lake Biwa with 100 m in maximum depth has worsened in recent years not only due to eutrophication in water but also the effects of global warming such as the rise of air temperature during winter. The vertical distribution of DO with a hollow or a minimum value in the thermo-cline between epilimnion and hypolimnion is observed during summer and autumn. This paper describes the generation mechanism of this typical DO distribution using DO transport equation with production, consumption and diffusion terms.