The dissolution of Carbon Dioxide (CO_2) drops covered with CO_2 hydrate films under a deep-sea condition has been investigated with a large high-pressure tank to evaluate environmental impacts of CO_2 ocean storage. Observation of CO_2 drops showed the dissolution of CO_2 drops was enhanced by disturbance around the drops, and pH measurement suggested dissolving CO_2 provided a CO_2-rich water layer above CO_2 drops. The drop diameter change was calculated by Ranz-Marshall's equation for a drop in flow with slow velocities and compared with the experimental result, which showed considerable dissolution rate would be provided by flow and increased with an increase in flow velocity.