Shogakukan (小学館 Shōgakukan?) is a Japanese publisher of dictionaries, literature, manga, non-fiction, DVDs, and other media in Japan.
Shogakukan founded Shueisha which founded Hakusensha. These are three separate companies, but are together called the Hitotsubashi Group, one of the largest publishing groups in Japan. Shogakukan is located in the Shogakukan Building in Hitotsubashi, Chiyoda, Tokyo, and the other two companies are located in the same ward.
Source: Wikipedia, "Shogakukan", available under the CC-BY-SA License.