An ultraviolet sterilizer having a watertight function, comprising: a housing having an inlet through which ballast water is drawn into the housing, and an outlet through which the ballast water is discharged from the housing after the ballast water has been sterilized; an ultraviolet sterilization unit provided in the housing, the ultraviolet sterilization unit comprising an ultraviolet lamp applying ultraviolet rays to the ballast water to sterilize the ballast water; and a cap supporting each of opposite ends of the ultraviolet sterilization unit, the cap being watertightly coupled to the housing, so that when the ultraviolet sterilization unit is damaged, ballast water is prevented from being drawn into a reception space formed by a housing cover and containing external devices located outside the cap, such as a drive unit and a power supply unit for the ultraviolet lamp, characterized in that the cap comprises: a conductor connected to the ultraviolet lamp to supply power to the ultraviolet lamp, the conductor being made of electric conductive material; and an insulator covering an outer surface of the conductor, the insulator being made of insulation material, and the conductor comprises: a conductor body forming an external shape of the conductor; and upper and lower wire connectors respectively protruding from upper and lower ends of the conductor body, wherein the lower wire connector is disposed inside the cap and connected to a lead wire of the ultraviolet lamp, and the upper wire connector is located outside the cap and connected to a power supply, the power supply supplying power to the ultraviolet lamp, so that a separate passing hole through which a lead wire of the ultraviolet lamp is led out of the cap is not present. a housing having an inlet through which ballast water is drawn into the housing, and an outlet through which the ballast water is discharged from the housing after the ballast water has been sterilized; an ultraviolet sterilization unit provided in the housing, the ultraviolet sterilization unit comprising an ultraviolet lamp applying ultraviolet rays to the ballast water to sterilize the ballast water; and a cap supporting each of opposite ends of the ultraviolet sterilization unit, the cap being watertightly coupled to the housing, so that when the ultraviolet sterilization unit is damaged, ballast water is prevented from being drawn into a reception space formed by a housing cover and containing external devices located outside the cap, such as a drive unit and a power supply unit for the ultraviolet lamp, characterized in that the cap comprises: a conductor connected to the ultraviolet lamp to supply power to the ultraviolet lamp, the conductor being made of electric conductive material; and an insulator covering an outer surface of the conductor, the insulator being made of insulation material, and the conductor comprises: a conductor body forming an external shape of the conductor; and upper and lower wire connectors respectively protruding from upper and lower ends of the conductor body, wherein the lower wire connector is disposed inside the cap and connected to a lead wire of the ultraviolet lamp, and the upper wire connector is located outside the cap and connected to a power supply, the power supply supplying power to the ultraviolet lamp, so that a separate passing hole through which a lead wire of the ultraviolet lamp is led out of the cap is not present. a conductor connected to the ultraviolet lamp to supply power to the ultraviolet lamp, the conductor being made of electric conductive material; and an insulator covering an outer surface of the conductor, the insulator being made of insulation material, and the conductor comprises: a conductor body forming an external shape of the conductor; and upper and lower wire connectors respectively protruding from upper and lower ends of the conductor body, wherein the lower wire connector is disposed inside the cap and connected to a lead wire of the ultraviolet lamp, and the upper wire connector is located outside the cap and connected to a power supply, the power supply supplying power to the ultraviolet lamp, so that a separate passing hole through which a lead wire of the ultraviolet lamp is led out of the cap is not present. a conductor body forming an external shape of the conductor; and upper and lower wire connectors respectively protruding from upper and lower ends of the conductor body, wherein the lower wire connector is disposed inside the cap and connected to a lead wire of the ultraviolet lamp, and the upper wire connector is located outside the cap and connected to a power supply, the power supply supplying power to the ultraviolet lamp, so that a separate passing hole through which a lead wire of the ultraviolet lamp is led out of the cap is not present.