As a step in investigating the possibility of storing CO2 in the deep sea, simulations on sedimentation of CO2 or CO2 clathrate were performed with consideration for dissolution. (1) In the case that CO2 is injected into the sea, it is necessary to inject CO2 at an extremely close point to the sea bottom, because CO2 is dissolved and diffused into the sea in a short time and it will affect the characteristics of the sea water in the vicinty of the injected point. (2) If the difference between injected depth and expected depth for clathrate storage is small, some part of the injected clathrate will be stored at the sea bottom, and then it is diffused into the sea to affect the characteristics of the sea water in the vicinity of the sea bottom. If the differnce is large, all injected clathrates are dissolved into sea water. Then it is expected that the effect of clathrate injection on the environment of deep sea would be small because the dissolved CO2 is diluted into the sea at each depth. (3) In the case that CO2 is stored at the sea bottom, it is necessary to be stored in a hollow place where CO2 is not dissolved into the sea from stored CO2 by diffusion on the basis of current flow, and pH is not partially increased to affect the vicinity in the neighborhood of stored place.