Abstract: A method for transmitting specific data whose data format is unknown at a relay from a first blockchain network to a second blockchain network via the relay is provided. The method includes steps of: the relay (a) in response to detecting that a blockchain communication message for transmitting the specific data from the first blockchain network following a first data format to the second blockchain network following a second data format is generated, generating a relay message including the blockchain communication message and verification information; and (b) transmitting the relay message to the second blockchain network, thereby instructing the second blockchain network to (i) verify the relay message by using the verification information included in the relay message and (ii) convert the specific data included in the relay message into the second data format to generate converted specific data and then transmit the converted specific data to a receiving party.