An agreement apparatus P(i) (where i=0, . . . , n−1) which executes a consensus protocol generates an opinion value with a signature Xij=(xi, sig_i(xi)) including an opinion value xi indicating an opinion and a signature sig_i(xi) on the opinion value xi or information different from the opinion value with the signature Xij as an opinion value with a signature X′ij=(x′ij, e′ij) and outputs the opinion value with the signature X′ij to an agreement apparatus P(j) (where j=0, . . . , n−1, i≠j). The agreement apparatus P(j) accepts the opinion value with the signature X′ij and outputs the opinion value with the signature X′ij or information different from the opinion value with the signature X′ij to an agreement apparatus P(m) (where m=0, . . . , n−1, m≠i, m≠j) as an opinion value with a signature X″ij.