Constant monitoring system was established in Osaka Bay in April of 2010. Seven of 13 stations can monitor water quality, but are located near a northeast coastline of the bay. The spatial resolution of the monitoring water quality is not sufficient in broad eastern part of the bay where hypoxia and excess growth of phytoplankton occurs. In the present study, the obtained data are verified by field survey with a boat and used to clarify a behavior of hypoxic water. The data unveils that vertical mixing occurs in the innermost area during 24 hours of a southerly strong wind, resulting in the hypoxic water disappears. External drift kriging with the numerically simulated results by three-dimensional water quality model ROMS (Regional Ocean Modeling System) is tested and improve the spatial interpolation.