In 1846, with the death of Naomoto II (the 11th son of Naonaka and Naosuke's elder brother), who was the heir of the 14th lord Naoaki (the third son of Naonaka and Naosuke's elder brother), therefore, Naosuke (the 14th son of Naonaka) was adopted as the heir to the Hikone Domain and appointed to Jushiino Jiju (Junior Fourth Rank Chamberlain) and Genba-ryo (the Office of Diplomacy and Buddhism).
ところが弘化3年（1846年）、第14代藩主で兄の直亮の世子であった井伊直元（直中十一男、これも兄にあたる）が死去したため、兄の養子という形で彦根藩の後継者に決定し、従四位侍従兼玄蕃寮に叙位・任官する。