is a 1975 Japanese manga by Keiko Takemiya. Natsu e no Tobira is a translation into Japanese of the title of the novel "The Door into Summer" by Robert A. Heinlein, though the content is quite different. It is an early example of a shōnen-ai manga. It was originally serialized in Hana to Yume, published by Hakusensha, and it was re-released in 2000 by Kodansha. An animated film version of this manga was released on March 20, 1981 in Japan.