A pollutant reduction device comprising: an exhaust gas pipe which discharges exhaust gas of a combustion engine; a seawater supply pipe which receives seawater from the outside and supplies ballast water to a ballast water tank and a pump; a cleaning water supply pipe which supplies cleaning water, and which branches from the seawater supply pipe; a scrubber which sprays the seawater supplied through the cleaning water supply pipe to the exhaust gas introduced through the exhaust gas pipe; a gas discharge pipe from the scrubber; a cleaning water discharge pipe which discharges the seawater inside the scrubber and supplies the seawater to the ballast water tank, the cleaning water discharge pipe being connected back to the seawater supply pipe by a filter unit; a purification unit which is connected to the exhaust gas pipe and the scrubber to supply an oxidizing agent, a neutralizing agent or a sterilizing agent to oxidize nitrogen-based oxides in the exhaust gas, or kill microorganisms present in the seawater, the purification unit being a device that produces the above by electrolyzing seawater or a device that produces the above by using plasma; a filter unit which filters the seawater which has been discharged through the cleaning water discharge pipe by using at least one of a centrifuge and a gravity separator, the filter unit connected to the seawater supply pipe between the pump and a control valve, so that the seawate supplied from the seawater supply pipe may be supplied to the scrubber via the filter unit and the cleaning water passing through the scrubber may pass through the filter again; and a three-way control valve installed at a connection portion between the cleaning water supply pipe and the seawater supply pipe, the three-way control-waive controls the amount of the filtered seawater which has been supplied to the scrubber through the cleaning water supply pipe or adjusts a ratio of seawater supplied to the cleaning water supply pipe to seawater supplied to a ballast water tank through the water supply pipe, a circulation pipe which connects the cleaning water discharge pipe and the cleaning water supply pipe, wherein the cleaning water discharged through the cleaning water discharge pipe is recirculated to the cleaning water supply pipe through the circulation pipe, wherein the device allows to recycle the filtered seawater to remove pollutants in the exhaust gas by supplying the filtered seawater to the scrubber. an exhaust gas pipe which discharges exhaust gas of a combustion engine; a seawater supply pipe which receives seawater from the outside and supplies ballast water to a ballast water tank and a pump; a cleaning water supply pipe which supplies cleaning water, and which branches from the seawater supply pipe; a scrubber which sprays the seawater supplied through the cleaning water supply pipe to the exhaust gas introduced through the exhaust gas pipe; a gas discharge pipe from the scrubber; a cleaning water discharge pipe which discharges the seawater inside the scrubber and supplies the seawater to the ballast water tank, the cleaning water discharge pipe being connected back to the seawater supply pipe by a filter unit; a purification unit which is connected to the exhaust gas pipe and the scrubber to supply an oxidizing agent, a neutralizing agent or a sterilizing agent to oxidize nitrogen-based oxides in the exhaust gas, or kill microorganisms present in the seawater, the purification unit being a device that produces the above by electrolyzing seawater or a device that produces the above by using plasma; a filter unit which filters the seawater which has been discharged through the cleaning water discharge pipe by using at least one of a centrifuge and a gravity separator, the filter unit connected to the seawater supply pipe between the pump and a control valve, so that the seawate supplied from the seawater supply pipe may be supplied to the scrubber via the filter unit and the cleaning water passing through the scrubber may pass through the filter again; and a three-way control valve installed at a connection portion between the cleaning water supply pipe and the seawater supply pipe, the three-way control-waive controls the amount of the filtered seawater which has been supplied to the scrubber through the cleaning water supply pipe or adjusts a ratio of seawater supplied to the cleaning water supply pipe to seawater supplied to a ballast water tank through the water supply pipe, a circulation pipe which connects the cleaning water discharge pipe and the cleaning water supply pipe, wherein the cleaning water discharged through the cleaning water discharge pipe is recirculated to the cleaning water supply pipe through the circulation pipe, wherein the device allows to recycle the filtered seawater to remove pollutants in the exhaust gas by supplying the filtered seawater to the scrubber.