The International Maritime Organization (IMO) has been adopted the International Convention for the Control and Management of Ship's Ballast Water and Sediments (the Ballast Water Management Convention) in 2004. The atmospheric pressure non-equilibrium discharge is useful as one of the plasma generation methods that are the most suitable, when it adapts to energy and environmental study. By irradiating the plasma to the seawater surface, the possibilities of ballast processing were examined. As the effect, it was proven that pH value decreases. The following factors effect the pH value: 1) The reaction with the radical. 2) The bond dissociation by the collision with electrons or ions. It is considered from the results that there is a possibility of ballast processing by plasma irradiation. However, it is necessary to carry out further verification.