An object of the present invention is to efficiently sterilize microorganisms present in or on a surface of a liquid. Plasma is generated in a vicinity of or in a manner to make contact with a liquid whose pH value is adjusted to become 4.8 or lower, more preferably 4.5 or lower. The plasma is generated in an atmospheric gas containing nitrogen, e.g., in the air. Superoxide anion radicals (O 2 - €¢) that are generated by the plasma react with protons (H + ) in the liquid to form hydroperoxy radicals (HOO€¢). Further, nitrogen and oxygen included in the air are combined together by the action of plasma to form nitrogen oxide such as nitric oxide (NO€¢). The nitric oxide (NO€¢) combines with the hydroperoxy radicals (HOO€¢) to become peroxynitrite (HOONO 2 ) having a high microbiocidal activity.
