Lower to middle tier regions of dam reservoirs often fall into the anoxic states in summer. When the anoxic state is reached, living things die, heavy metals such as manganese and iron, hydrogen sulfide and methane are eluted from mud deposited on the lakebed. Since it is not possible to release heavy metals exceeding environmental standards, an improvement of dissolved oxygen concentration in dam lakebed is required. So far, there is no system that can automatically improve the dissolved oxygen, in the lower to middle tier regions of dam lakebed. This study was carried out for developing an automatic system to improve the lakebed dissolved oxygen concentration to 6mg/L or more. The developed oxygen dissolution apparatus can diffuse a high concentration oxygen water horizontally at a rate of 1km per day. Thus, by raising and lowering this oxygen dissolution apparatus in water, a system that creates 3 - dimensional spreading of the dissolved oxygen not only to the lower tier but also to the middle tier regions was developed. The demonstration experiments, reported of the effectiveness of the system that can raise the dissolved oxygen concentration from the low oxygen-free state to the target of 10 mg / L in 5 days. Moreover, the system demonstrated its ability to automatically raise the dissolved oxygen concentration to 10mg/L in the 10m region from the bottom of lakebed.