An optical isolator including: (a) a support body; (b) an optical fiber assembly disposed in the support body including a first single mode fiber, a first graded index fiber, a coreless fiber, a second graded index fiber, and a second single mode fiber; and (c) an optical isolator element disposed in a groove formed in the support body so as to separate the coreless fiber. The optical isolator element is tilted with respect to the optical axis of the optical fiber assembly, thereby to achieve optical coupling with both the end face of the first graded index fiber on the coreless fiber side and the end face of said the graded index fiber on the coreless fiber side.