An ultraviolet ballast water treatment apparatus having a rectangular cross-section, comprising: a body including an inlet port and an outlet port through which ballast water is drawn into and discharged out of the body; an ultraviolet lamp unit including ultraviolet lamps applying ultraviolet rays to ballast water flowing through the body; and a washing unit provided to remove foreign substances adhering to the ultraviolet lamp unit, wherein a cross-section of the body that is perpendicular to a direction of orientation of the ultraviolet lamps is rectangular, and the ultraviolet lamps are arranged at regular intervals in the rectangular cross-section, whereby a number of ultraviolet lamps arranged per a unit cross-sectional area is minimized, wherein the ultraviolet lamps are arranged in a plurality of lines (330; 330-1, 330-2, 330-3, 330-4, 330-5) in the rectangular cross-section of the body, each of the lines (330; 303-1, 330-2, 330-3, 330-4, 330-5) including the corresponding ultraviolet lamps arranged in a row perpendicular to a direction in which the ballast water flows, wherein the ultraviolet lamps of a second line spaced apart from a first line adjacent to the inlet port by a predetermined distance with respect to the direction of the flow of ballast water are respectively disposed between the ultraviolet lamps of the first line when seen in the direction of the flow of ballast water, thereby preventing the ballast water from being discharged out of the body without colliding with the ultraviolet lamps, wherein the lines (330; 330-1, 330-2, 330-3, 330-4, 330-5) comprise three lines, and wherein a drive shaft provided to move an arm provided with a wiper is disposed on an intermediate line of the three lines, the wiper being used to remove foreign substances adhering to the ultraviolet lamp unit, and wherein the ultraviolet lamp of the intermediate line that corresponds to a position interfering with the drive shaft is disposed ahead of or behind the drive shaft with respect to the direction of the flow of the ballast water, whereby the ballast water is prevented from being discharged out of the body without colliding with the ultraviolet lamps. a body including an inlet port and an outlet port through which ballast water is drawn into and discharged out of the body; an ultraviolet lamp unit including ultraviolet lamps applying ultraviolet rays to ballast water flowing through the body; and a washing unit provided to remove foreign substances adhering to the ultraviolet lamp unit, wherein a cross-section of the body that is perpendicular to a direction of orientation of the ultraviolet lamps is rectangular, and the ultraviolet lamps are arranged at regular intervals in the rectangular cross-section, whereby a number of ultraviolet lamps arranged per a unit cross-sectional area is minimized, wherein the ultraviolet lamps are arranged in a plurality of lines (330; 330-1, 330-2, 330-3, 330-4, 330-5) in the rectangular cross-section of the body, each of the lines (330; 303-1, 330-2, 330-3, 330-4, 330-5) including the corresponding ultraviolet lamps arranged in a row perpendicular to a direction in which the ballast water flows, wherein the ultraviolet lamps of a second line spaced apart from a first line adjacent to the inlet port by a predetermined distance with respect to the direction of the flow of ballast water are respectively disposed between the ultraviolet lamps of the first line when seen in the direction of the flow of ballast water, thereby preventing the ballast water from being discharged out of the body without colliding with the ultraviolet lamps, wherein the lines (330; 330-1, 330-2, 330-3, 330-4, 330-5) comprise three lines, and wherein a drive shaft provided to move an arm provided with a wiper is disposed on an intermediate line of the three lines, the wiper being used to remove foreign substances adhering to the ultraviolet lamp unit, and wherein the ultraviolet lamp of the intermediate line that corresponds to a position interfering with the drive shaft is disposed ahead of or behind the drive shaft with respect to the direction of the flow of the ballast water, whereby the ballast water is prevented from being discharged out of the body without colliding with the ultraviolet lamps.