A battery monitoring system includes a monitoring unit that monitors a battery and a higher device that is provided with monitoring data obtained by monitoring of the battery by the monitoring unit and saves, by using a blockchain, at least part of specific information included in the monitoring data. The higher device includes a block generation unit that generates a new block including a hash value based on the at least part of the specific information and a last block linked at an end of the blockchain, and an external communication device configured to save, in a distributed manner in a plurality of external devices, the new block generated by the block generation unit. At least one of the monitoring unit or the higher device is provided with a data protection unit configured to ensure reliability of the monitoring data.