PURPOSE: To provide an attaching tool which aligns and fixes the optical fiber, including a core surrounded with a coating layer and having an end part with a grade, to an optical device which has an active area larger than the section of the core of the optical fiber.

CONSTITUTION: An attaching tool 11 includes a base 12 having an internally demarcated receptacle 30. The receptacle 30 is extended throughout the base from a first side face 32 to a second side face 22 and is practically larger than the section of the optical fiber in the first side face. The receptcle 30 is smaller than the section of the optical fiber 14 and is at least as large as the section of a core 16 of the optical fiber in the second side face 22. The base 12 demarcates a taper area 40, which has a grade approximately equal to the grade in the end part of the optical fiber, in the receptacle 30.