A ballast water treatment method includes: a step of supplying a sterilizing component to a ballast pipe while taking ballast water into a ballast tank through the ballast pipe; a first measurement step of measuring the concentration of the sterilizing component in the ballast water after the sterilizing component is supplied; a circulation step of returning the ballast water stored in the ballast tank to the ballast pipe through a circulation pipe; a second measurement step of measuring the concentration of the sterilizing component contained in the ballast water returned to the ballast pipe; and a step of supplying the sterilizing component to the ballast pipe 2 when the concentration of the sterilizing component measured in the second measurement step is less than 0.2 times the concentration of the sterilizing component measured in the first measurement step. In the first supply step, the sterilizing component is supplied to the ballast pipe 2 such that the concentration of the sterilizing component measured in the first measurement step becomes 6 mg/L or more.