He made his professional debut as a manga artist in 1952 with his story Sansho no Piri-chan (さんしょのピリちゃん).

While there, he formed the group Musashi Production with artists including Atsushi Sugimura (who was working under the pseudonym Kontarō), Kyūta Ishikawa and Kuni Fukai (who was working under the pseudonym Hirō Fukai).

He soon became friends with several members of the Gekiga Kōbō, including Yoshihiro Tatsumi and Takao Saito, while living in a provincial temple.

In 1961, Nagashima published The Harsh Story of a Manga Artist (漫画家残酷物語, Mangaka Zankoku Monogatari), a story which showed the "other side" of the manga industry and which brought Nagashima to the forefront of that industry.

[1] He continued publishing new works in a variety of magazines such as COM and Garo, and due to his unusual style began to be called the "father of seinen manga".