An ultraviolet sterilizer having a vibration-proof 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 therefrom after the ballast water has been sterilized; an ultraviolet sterilization unit provided inside the housing, the ultraviolet sterilization unit including an ultraviolet lamp applying ultraviolet rays to the ballast water to sterilize the ballast water; a cap supporting each of opposite ends of the ultraviolet sterilization unit, wherein the cap has an air inlet hole formed through a predetermined portion of the cap; a shock absorption unit having a first end elastically compressed by the cap and a second end elastically compressing a corresponding end of the ultraviolet lamp, whereby the ultraviolet lamp is prevented from being damaged by a vibration; and a pressure controller connected to the predetermined portion of the cap, the pressure controller supplying air into the cap such that an internal pressure of the cap is maintained higher than an external pressure thereof, and the pressure controller includes a connector coupled to the air inlet hole, an air supply tube connected at a first end thereof to the connector, an air supply unit connected to a second end of the air supply tube, the air supply unit generating air pressure to supply air into the air supply tube, and a backflow prevention valve provided at a predetermined position in the air supply tube to prevent ballast water from flowing backwards from the cap towards the air supply unit. a housing having an inlet through which ballast water is drawn into the housing, and an outlet through which the ballast water is discharged therefrom after the ballast water has been sterilized; an ultraviolet sterilization unit provided inside the housing, the ultraviolet sterilization unit including an ultraviolet lamp applying ultraviolet rays to the ballast water to sterilize the ballast water; a cap supporting each of opposite ends of the ultraviolet sterilization unit, wherein the cap has an air inlet hole formed through a predetermined portion of the cap; a shock absorption unit having a first end elastically compressed by the cap and a second end elastically compressing a corresponding end of the ultraviolet lamp, whereby the ultraviolet lamp is prevented from being damaged by a vibration; and a pressure controller connected to the predetermined portion of the cap, the pressure controller supplying air into the cap such that an internal pressure of the cap is maintained higher than an external pressure thereof, and the pressure controller includes a connector coupled to the air inlet hole, an air supply tube connected at a first end thereof to the connector, an air supply unit connected to a second end of the air supply tube, the air supply unit generating air pressure to supply air into the air supply tube, and a backflow prevention valve provided at a predetermined position in the air supply tube to prevent ballast water from flowing backwards from the cap towards the air supply unit. a connector coupled to the air inlet hole, an air supply tube connected at a first end thereof to the connector, an air supply unit connected to a second end of the air supply tube, the air supply unit generating air pressure to supply air into the air supply tube, and a backflow prevention valve provided at a predetermined position in the air supply tube to prevent ballast water from flowing backwards from the cap towards the air supply unit.