A monitoring device configured to monitor a lightning arrester including a plurality of elements arranged in a sealed tank, the monitoring device comprising: a detecting unit to detect an internal state of the sealed tank; a damage determining unit to determine damage of an element due to a lightning stroke on the basis of a detection result detected by the detecting unit; and a processing determining unit to determine whether or not the lightning stroke is processed after the element is determined to be damaged, wherein the detecting unit is a gas pressure detecting unit to detect gas pressure in the sealed tank, the gas pressure being the internal state, characterized in that the monitoring device further comprises a system protecting unit to monitor current and voltage of a system connected with the lightning arrester, cut off power supply to the system in a case of a lightning stroke, and restore power supply to the system after the cut off, wherein when a value of an increase in the gas pressure in the sealed tank within a preset time exceeds a threshold being a criterion for element damage, the damage determining unit determines that the element is damaged by a lightning stroke, when the power supply to the system is restored by the system protecting unit, the processing determining unit determines that the lightning stroke is processed, and the monitoring device further comprises: a partial damage determining unit to determine that some of the elements are damaged when the element is determined to be damaged by the damage determining unit and the lightning stroke is determined to have been processed by the processing determining unit, and to determine that all or most of the elements are damaged when the element is determined to be damaged by the damage determining unit and the lightning stroke is determined not to have been processed by the processing determining unit. a detecting unit to detect an internal state of the sealed tank; a damage determining unit to determine damage of an element due to a lightning stroke on the basis of a detection result detected by the detecting unit; and a processing determining unit to determine whether or not the lightning stroke is processed after the element is determined to be damaged, wherein the detecting unit is a gas pressure detecting unit to detect gas pressure in the sealed tank, the gas pressure being the internal state, characterized in that the monitoring device further comprises a system protecting unit to monitor current and voltage of a system connected with the lightning arrester, cut off power supply to the system in a case of a lightning stroke, and restore power supply to the system after the cut off, wherein when a value of an increase in the gas pressure in the sealed tank within a preset time exceeds a threshold being a criterion for element damage, the damage determining unit determines that the element is damaged by a lightning stroke, when the power supply to the system is restored by the system protecting unit, the processing determining unit determines that the lightning stroke is processed, and the monitoring device further comprises: a partial damage determining unit to determine that some of the elements are damaged when the element is determined to be damaged by the damage determining unit and the lightning stroke is determined to have been processed by the processing determining unit, and to determine that all or most of the elements are damaged when the element is determined to be damaged by the damage determining unit and the lightning stroke is determined not to have been processed by the processing determining unit.