A ballast water treatment device comprises: a cylindrical pleated filter held so as to be capable of rotating centered on the axis of the cylinder; a driving mechanism for rotating the pleated filter; a treatment water nozzle for causing the water to be treated to flow toward the outer circumferential surface of the pleated filter; a case provided so as to surround the pleated filter and having an outer cylindrical section equipped therein with the nozzle spout of the treatment water nozzle; a filtered water flow channel for leading the filtered water that has passed through the pleated filter out from the inside of the pleated filter cylinder to the outside of the case; and a discharge flow channel for discharging effluent that has not been filtered by the pleated filter to the outside of the case. The ballast water treatment device is provided with a back-washing mechanism for supplying rinsing water to the inside of the pleated filter cylinder through the filtered water flow channel.