In this paper, we present numerical study on the rise of pCO2 in seawater column on continental shelf by the leakage of CO2, which is purposefully stored in the aquifer under the seabed, in an extreme case. The target in this study was limited to the ocean above the seabed and does not include subsea underground. First, parameter studies were conducted in a small domain to see the impacts of CO2 leakage and seawater conditions. In addition, a more realistic case simulation was also conducted with topography and tidal current around a Japanese coast. As a result, it was suggested that the rise of pCO2 is smaller than the so-called predicted non-effective concentration for marine organisms, such as zooplanktons, even in such an extreme case.