A multi-core fiber comprising: six or more of core elements including: a core ; a first clad surrounding an outer circumferential surface of the core ; and a second clad surrounding an outer circumferential surface of the first clad ; and a clad surrounding the core elements , wherein: all of expressions are satisfied : n 1 > n 2 > n 3 n 1 > n 4 n 3 < n 4 wherein a refractive index of the core is n 1 , a refractive index of the first clad is n 2 , a refractive index of the second clad is n 3 , and a refractive index of the clad is n 4 ; and the core elements are disposed so that an inter-center pitch of A microns between the cores adjacent to each other is disposed at regular spacings and centers of the cores are annularly disposed and wherein all cores are disposed on an annulus, characterized in that an expression is satisfied: A eff 1 2 Λ 2 ≥ 0.0001 XT + 0.01 wherein a crosstalk between the core and the adjacent core when light propagates through the cores for 100 km is XT dB, and effective areas of the cores are A eff micron 2 , wherein A eff is between 80 and 110, wherein the crosstalk XT in the multi-core fiber is calculated by an expression : XT = 2 κ 2 β R Λ L wherein L is a length of the multi-core fiber , R is a radius of the multi-core fiber in bent state, K is a coupling coefficient of the cores adjacent to each other, and β is a propagation constant of the cores . six or more of core elements including: a core ; a first clad surrounding an outer circumferential surface of the core ; and a second clad surrounding an outer circumferential surface of the first clad ; and a core ; a first clad surrounding an outer circumferential surface of the core ; and a second clad surrounding an outer circumferential surface of the first clad ; and a clad surrounding the core elements , wherein: all of expressions are satisfied : n 1 > n 2 > n 3 n 1 > n 4 n 3 < n 4 wherein a refractive index of the core is n 1 , a refractive index of the first clad is n 2 , a refractive index of the second clad is n 3 , and a refractive index of the clad is n 4 ; and the core elements are disposed so that an inter-center pitch of A microns between the cores adjacent to each other is disposed at regular spacings and centers of the cores are annularly disposed and wherein all cores are disposed on an annulus, characterized in that an expression is satisfied: A eff 1 2 Λ 2 ≥ 0.0001 XT + 0.01 wherein a crosstalk between the core and the adjacent core when light propagates through the cores for 100 km is XT dB, and effective areas of the cores are A eff micron 2 , wherein A eff is between 80 and 110, wherein the crosstalk XT in the multi-core fiber is calculated by an expression : XT = 2 κ 2 β R Λ L wherein L is a length of the multi-core fiber , R is a radius of the multi-core fiber in bent state, K is a coupling coefficient of the cores adjacent to each other, and β is a propagation constant of the cores . wherein A eff is between 80 and 110, wherein the crosstalk XT in the multi-core fiber is calculated by an expression : XT = 2 κ 2 β R Λ L wherein L is a length of the multi-core fiber , R is a radius of the multi-core fiber in bent state, K is a coupling coefficient of the cores adjacent to each other, and β is a propagation constant of the cores . <EOS>