An authentication server authenticates a voter using authentication data including a first identifier (ID) associated with the voter and authentication information regarding the voter. The authentication server further synchronizes a first blockchain. The first blockchain includes first transaction data indicating that the voter has been authenticated with one or more of other authentication servers. A voting server receives, from a terminal, voting data including a second ID associated with a vote cast by the voter and voting information indicating the vote. The voting server synchronizes a second blockchain including, as second transaction data, the voting information included in the voting data with one or more of other voting servers. The terminal transmits the authentication data to the authentication server and, after the authentication is successfully completed, transmits the voting data to the voting server.