A ballast water treatment apparatus includes a discharge path for discharging ballast water from a ballast tank that stores the ballast water, an ultraviolet irradiation device that includes an ultraviolet lamp and that irradiates the ballast water passing through the discharge path with ultraviolet light, a circulation path formed so that the ballast water circulates therethrough and is irradiated with ultraviolet light emitted from the ultraviolet lamp, a switching portion for switching a path through which the ballast water flows between the circulation path and the discharge path, and a control device that controls the switching portion so that the ballast water flows through the circulation path during a switching-on process of the ultraviolet lamp and controls the switching portion so that the ballast water flows through the discharge path when the switching on of the ultraviolet lamp is completed.