Nogizaka46 member Nakamoto Himeka announced her graduation in NHK radio show "Rajira! Sunday" on 6 August 2017, she did not reveal her graduation date. 

Nakamoto Himeka took a hiatus from January to April 2017 because of poor health condition.

Nakamoto Himeka is Nogizaka46 1st generation, she joined Nogizaka46 in 2011, she had been a member of the idol group "SPL∞ASH" before joining Nogizaka46, and her younger sister is Nakamoto Suzuka of the idol group "BABYMETAL".

Reference: Nogizaka46 official site
http://www.nogizaka46.com/news/2017/08/06-nakamoto.php