A system for monitoring a state of a wind turbine generator which comprises at least one blade, the system comprising: a lightning strike parameter acquisition part configured to acquire at least one lightning strike parameter which indicates a level of the lightning strike to the at least one blade; a first buffer configured to receive the lightning strike parameter from the lightning strike parameter acquisition part and overwrite data regarding the lightning strike parameter within a first record period so as to accumulate the data temporarily; a first trigger signal output part configured to output a first trigger signal when the lightning strike parameter exceeds a threshold; a memory part configured to acquire at least a part of the data regarding the lightning strike parameter of a period from the first buffer based on the first trigger signal, and store the at least part of the data, said period being at least a part of the first record period and including before and after an output time of the first trigger signal; a reference value estimation part configured to estimate a reference value of the lightning strike parameter based on a first data corresponding to a first period before the output time of the first trigger signal among the data stored in the memory part; and a damage state estimation part configured to estimate a damage state of the at least one blade based on a second data corresponding to a second period among the data stored in the memory part by considering the reference value, the second period being after the output time of the first trigger signal, wherein the damage state estimation part is configured to estimate the damage state based on a difference between the lightning strike parameter of the second data and the reference value. a lightning strike parameter acquisition part configured to acquire at least one lightning strike parameter which indicates a level of the lightning strike to the at least one blade; a first buffer configured to receive the lightning strike parameter from the lightning strike parameter acquisition part and overwrite data regarding the lightning strike parameter within a first record period so as to accumulate the data temporarily; a first trigger signal output part configured to output a first trigger signal when the lightning strike parameter exceeds a threshold; a memory part configured to acquire at least a part of the data regarding the lightning strike parameter of a period from the first buffer based on the first trigger signal, and store the at least part of the data, said period being at least a part of the first record period and including before and after an output time of the first trigger signal; a reference value estimation part configured to estimate a reference value of the lightning strike parameter based on a first data corresponding to a first period before the output time of the first trigger signal among the data stored in the memory part; and a damage state estimation part configured to estimate a damage state of the at least one blade based on a second data corresponding to a second period among the data stored in the memory part by considering the reference value, the second period being after the output time of the first trigger signal, wherein the damage state estimation part is configured to estimate the damage state based on a difference between the lightning strike parameter of the second data and the reference value.