What is the most popular novel in Japan?

Kokoro by Natsume Sōseki. First published in 1914, Kokoro is considered to be one of the greatest novels in Japanese literature. It tells the story of a young man who meets and befriends a mysterious older man named Sensei. Over the course of their relationship, Sensei reveals his own tragic past and his complex inner world. Kokoro has been translated into many languages and has been adapted into several films and television dramas.