Okuchichibu Mountains (奥秩父山塊,Okuchichibu Sankai) or the Okuchichibu Mountainous Region (奥秩父山地,Okuchichibu Sanchi) is a mountainous district in the Kantō region and Kōshin'etsu region, Japan. It covers the western part of Tokyo, the western part of Saitama Prefecture, the southwestern part of Gunma Prefecture, the southeastern part of Nagano Prefecture, and the northern part of Yamanashi Prefecture. Oku (奥, oku) means the interior, Okuchichibu means the interior of chichibu (秩父, chichibu). The meaning of the word Okuchichibu is based on the point of view from the Kantō region. This mountain area consists of folded mountains and ranges from 1000 to 2600 meters in height. Mount Kita Okusenjō (北奥千丈岳,Kita Okusenjō-dake) is the highest at 2601m. Most of the range lies in the Chichibu Tama Kai National Park (秩父多摩甲斐国立公園,Chichibu Tama Kai Kokuritsu Kōen).