A practical treatment apparatus for ship ballast water, which can kill aquatic organisms contained in the ballast water, is provided by being easily incorporated in an existing ballast-water system piping in a ship. In a branch pipe 131 branched from the existing ballast-water system piping for taking in the ballast water into the ship by a first ballast pump 4 with high flow-rate/low pressure and transferring it to a ballast tank 2, an ozone mixing device for mixing ozone generated by an ozone generator, a second ballast pump 134 for discharging said ballast water with lower flow-rate/high pressure than those of said first ballast pump, a slit plate 135 provided on the secondary side of said second ballast pump 134 and having a plurality of slit-shaped openings for passing said ballast water, and a deaeration tank 136 for deaerating undissolved ozone from the ballast water are provided, and the apparatus is configured so that the ballast water after deaeration discharged from said deaeration tank 136 is returned to said ballast-water system piping.