In 1624, he was appointed teacher of the third shogun Iemitsu TOKUGAWA (the eldest son of Hidetada), and subsequently joined the bakufu (Japanese feudal government headed by a shogun) government.
寛永元年(1624年)3代将軍・徳川家光（秀忠の長男）の侍講になり、以後幕府政治にも関わっていくことになる。