In Islamic studies, Toshihiko Izutsu is well known as a scholar of the Qur’an and Sufism, thanks to his published works in English. As to his image in Japan, however, he is known as a thinker of Oriental philosophy. After his return from Iran to Japan, he published several publications on Oriental philosophy in Japanese. As such, his final achievement, which was his Oriental philosophy, was veiled from readers who do not know Japanese, while his detailed study on Islamic studies is not known by Japanese readers. In structuring his Oriental philosophy, he refers mainly to Ibn ‘Arabī’s philosophy known as waḥdat al-wujūd. Before publishing his work in English, he had already considered Ibn ‘Arabī and mysticism in his Japanese work in which he emphasizes the essential role of experience in understanding mysticism. Based on the theoretical development from mysticism to mystical philosophy, Izutsu delineates the theoretical development from Sufism to Islamic philosophy. In studying Ibn ‘Arabī’s philosophy, waḥdat al-wujūd or ‘Unity of Existence’, Izutsu uses Islamic philosophy as the framework for his Oriental philosophy.