The first island created by Izanagi and Izanami was Onogoro, also called Onogoroshima.
In the creation narrative of the Kojiki, the divine couple stand on the Floating Bridge of Heaven and churn the primeval sea with the jeweled spear Amenonuhoko. When they lift the spear, brine drips from its tip and congeals into Onogoro. They descend there, build a palace, and raise the Heavenly Pillar.
Onogoro is the setting for Izanagi and Izanami’s marriage rite and the beginning of their work creating the Japanese islands. Their first union produces the leech-child Hiruko, regarded as an unsuccessful birth, so they repeat the ceremony with Izanagi speaking first. Their later union leads to the birth of the archipelago and many kami.
Onogoro is a mythic island, not one of Japan’s four main modern islands. Later interpreters proposed real locations, especially Nushima near Awaji Island, where Onogoro-jinja commemorates the story. Shikoku, Kyushu, and Honshu are later islands in the creation sequence, not the first one.