There are quite a few manga magazines that started before Shonen Jump. Going off of this list, the earliest published manga magazine was Monthly Manga Shounen (Gekkan Manga Shounen), which started in 1947.
There are manga magazine publishing companies that predate 1947, such as Shogakukan, which was founded in 1922; Kodansha, which was formally founded in 1938; and Kadokawa Shoten, which was founded in 1945. As far as I can tell, though, none of these companies published manga magazines before 1947.
Toward the Terra was one manga published in Monthly Manga Shounen.