The present invention provides an optical fiber which can have a larger NA and a preferable mechanical strength even with a monolayer coating and can be fabricated at low cost, and which can transmit excitation light efficiently reducing a loss even under a high temperature environment during the operation of a fiber laser. An optical fiber according to an embodiment of the present invention includes a core, a glass cladding which is provided at a periphery of the core and has a refractive index smaller than the core, and a polymer cladding which is provided at a periphery of the glass cladding and has a refractive index smaller than the glass cladding. The polymer cladding contains fluorine and the polymer cladding has a difference between an elasticity modulus at 60° C. and that at 23° C. equal to or smaller than 100 MPa and also has an elasticity modulus equal to or larger than 200 MPa at 23° C.