A ships ballast water treatment agent is obtained by blending phosphate and sodium hydroxide into a hypochlorite solution and has pH of 10 to 13. A blending amount of phosphate is preferably such that a weight ratio of effective chlorine to phosphate acid (PO4) (effective chlorine:phosphate acid) is 30:1 to 2:1. A ships ballast water treatment agent as such has excellent performance in killing bacteria and microorganisms, such as plankton, included in ships ballast water and has stability and an anticorrosion property.
