The present invention relates to a multi-core fiber having a structure for effectively suppressing crosstalk increase between cores caused by bending within an allowable range. The multi-core fiber comprises a plurality of types of cores respectively extending along an optical axis and a cladding region, and the effective refractive index of each core is set so that, in all pairs of cores of different types, a relative refractive index difference between an effective refractive index of a core of a certain type and an effective refractive index of a core of another type satisfies a condition regulated according to a core spacing between cores and a bending radius.