Disclosed herein is an apparatus for sterilizing ballast water. Ultraviolet lamps are installed in a body, and a cross-section of the body that is perpendicular to the orientation of the ultraviolet lamps is rectangular. The ultraviolet lamps are arranged at regular intervals in the rectangular cross-section of the body so that the number of ultraviolet lamps arranged per a unit cross-sectional area can be minimized. Thereby, the power consumption of the apparatus and the space required for installation thereof can be reduced. Furthermore, lines, each of which includes ultraviolet lamps arranged in a row perpendicular to the direction of the flow of ballast water, are arranged in the rectangular cross-section of the body. The ultraviolet lamps of each line are disposed between the ultraviolet lamps of the preceding or following line. Thus, ballast water can be prevented from being discharged out of the body without colliding with any ultraviolet lamp.