A ballast-water treating apparatus that improves the performance of an in-line mixer so as to be able to sterilize a large volume of ballast water within a short period of time. There is provided a ballast-water treating apparatus that includes pipelines for feeding water to a ship's ballast tank(s) and for draining water from the ballast tank(s), ballast pumps on the pipelines, an ozone generator or hydrogen peroxide generator, a gas/liquid injection pipe for introducing generated ozone or hydrogen peroxide into the pipelines; and an in-line mixer that has two blades disposed crosswise in a tubular part composed of a main tubular part, a subsidiary tubular part that has an inside diameter smaller than that of the main tubular part and that is furnished with multiple projections, and a tapered part. The ballast-water treating apparatus can simultaneously carry out mechanical destructive sterilization via a swirling flow and sterilization of chemically active substances by micromixing.