Influence of seawater composition on electrolytic alkaline preparation was investigated for improving the efficiency of a wet scrubber that cleans exhaust gas from ships. Saline（NaCl 3.5%）, seawater, and 8L of simulated seawater were poured into a tank and electrolyzed. The pH level, alkalinity, and conductivity of solutions were measured. Both of the anode and the cathode were made of carbon. The compositions of solutions were also controlled. Results indicated that alkalinity was increased by the electrolysis but that the pH level was not. This was because CO32－ and Mg（OH）2 were generated by a reaction between HCO3－ and OH－, caused by cathode electrodes. It was also indicated that the increase of alkalinity was almost the same in all solutions due to CO32－ that formed from OH－ and HCO3－, contributing to the neutralization of acid.