The present invention relates to an apparatus and a method for treating ballast water. The object of the present invention is to provide an apparatus and a method for precisely controlling the production, injection, and removal of disinfectants using electrolysis according to the flow rate of ballast water, wherein the ballast water flows into a ballast tank or is discharged from the ballast tank in order to manage the ballast water of ships so that the destruction or disruption of marine ecosystems can be prevented. For the above-mentioned object, the present invention provides an apparatus for treating ballast water and a treatment method using the same, wherein the apparatus comprises: an electrolysis module (4) for producing concentration-controlled sodium hypochlorite; a gas-liquid separator (5) for separating hydrogen gas; a micro bubble generator (18) for mixing and supplying reducing agents while reducing residual chlorine by making micro bubbles in the ballast water; a vortex generator (19) for generating vortex; and a control system (12). The control system controls a salinometer (8), a flow rate meter (9), a residual chlorine meter (10), a seawater supply pump (2), a flow rate control valve (3), an electrolysis module (4), a residual chlorine meter (16), a micro bubble generator (18), and an injection pump (15).