The invention provides an optical fiber which is capable of restraining the deterioration of connection loss although changing the refractive index of the hardening resin because of using the change of the environmental temperature. The optical fiber (10) provided by the invention includes a core (11), a clad (12) having a plurality of air holes (13) at a periphery of the core (11), and a hardening resin (15) filled in a sealed portion (16) at an end of the plurality of air holes (13). The hardening resin (15) after hardening has a fill length of 7 mm to 8 mm.