Producing various carbonate by using cation such as Mg. Ca, Na originated from seawater and CO2 generated from a thermal power plant is attracting attention. Carbonate production has been actively utilized to CO2 storage by using this method. Problems arise in the magnesium hydroxide manufacturing process. The Mg dissolved in the seawater forms precipitation under the condition of the pH 10-12 by the addition of alkali, at the same time, calcium carbonate precipitation occurs. In order to prevent coprecipitation of CaCO3 in the preci-pitation of Mg(OH)2, careful control is needed for high separation factor of Ca/Mg. Various decarbonation methods such as acid treatment, heat treatment and addition of additives are used.