Ballast water treatment system and ballast water treatment method

Provided is a ballast water treatment system equipped with: a ballast water supply line (107) that connects a water intake port (104) and a ballast tank (103); an eradication treatment device (102) positioned on the line (107), for electrically or mechanically eradicating aquatic organisms in fluid taken from the water intake port (104); a chemical supply device (101) that is connected to the line (107), and supplies sodium hypochlorite to the line (107) in order to kill aquatic organisms in fluid taken from the water intake port (104). The chemical supply device (101) is connected to a second water intake port (114), which is different to the water intake port (104) to which the ballast water supply line (107) is connected, and produces sodium hypochlorite by electrolyzing fluid taken from the second water intake port (114).