in Japanese history, office of senior minister or chief councillor, the highest administrative post in the shogunate during the Tokugawa period (1603–1867). The office of tairō stood above the other senior councillors (rōjū), and so resembled the position of prime minister. Its chief function was to advise on matters of high policy or to serve as shogunal regent. After 1648, however, the office was filled only in times of crisis for a specific purpose. The best known figure to hold the position was Ii Naosuke, appointed tairō in 1858. Ii was largely responsible for the decision to sign a commercial treaty with the United States in 1858, soon followed by treaties with England, France, Russia, and The Netherlands, and he also acted swiftly to decide a succession dispute when the shogun Iesada died in the same year leaving no heir.