A consensus building method suitable when f Byzantine failure nodes, wherein f is an integer equal to or larger than 1 and smaller than N/3, are assumed in a network having N nodes, wherein N is an integer equal to or larger than 4, participating in consensus building, comprising the steps of: receiving a first message from other node which communicates that the other node determined a message including data subject to consensus building valid as a proposal, when the number of received first messages reach a predetermined value Q, transmitting a second message to each node which communicates that it is accepting the proposal, wherein the second message includes evidential data which is a basis for determining that the predetermined value Q was reached, and when the number of received first messages do not reach said predetermined value Q, transmitting a third message to each node which communicates that it is dismissing the proposal, when the number of received second messages reach a predetermined value Q, transmitting a fourth message to each node which communicates that it is treating the proposal as agreed in the network, wherein the fourth message includes data subject to consensus building or a hash value calculated from the data, wherein the data is a block or a hash value thereof, and wherein the block is added to a blockchain of each node when the number of received second messages reach a predetermined value Q; and when the number of received third messages reach a predetermined value Q, transmitting a fifth message for proceeding to a next round, wherein a unit of consensus building process is called "round", to each node, wherein each predetermined value Q is an integer equal to or larger than (f+N+1)/2 when a value of f is known, wherein when the number of received first message reaches a predetermined value Q, or a second message from the same round is received from other node, a lock is set to limit behaviors thereafter, and wherein the limit includes to make a proposal in which data subject to consensus building is same as the data subject to consensus building at the time when the lock was set when it is selected as a proposer node. receiving a first message from other node which communicates that the other node determined a message including data subject to consensus building valid as a proposal, when the number of received first messages reach a predetermined value Q, transmitting a second message to each node which communicates that it is accepting the proposal, wherein the second message includes evidential data which is a basis for determining that the predetermined value Q was reached, and when the number of received first messages do not reach said predetermined value Q, transmitting a third message to each node which communicates that it is dismissing the proposal, when the number of received second messages reach a predetermined value Q, transmitting a fourth message to each node which communicates that it is treating the proposal as agreed in the network, wherein the fourth message includes data subject to consensus building or a hash value calculated from the data, wherein the data is a block or a hash value thereof, and wherein the block is added to a blockchain of each node when the number of received second messages reach a predetermined value Q; and when the number of received third messages reach a predetermined value Q, transmitting a fifth message for proceeding to a next round, wherein a unit of consensus building process is called "round", to each node, wherein each predetermined value Q is an integer equal to or larger than (f+N+1)/2 when a value of f is known, wherein when the number of received first message reaches a predetermined value Q, or a second message from the same round is received from other node, a lock is set to limit behaviors thereafter, and wherein the limit includes to make a proposal in which data subject to consensus building is same as the data subject to consensus building at the time when the lock was set when it is selected as a proposer node.