A multi-core optical fiber (1a; 1b; 1c; 1; 2) applicable to a transmission line of a multi-core optical fiber transmission system with a transmission distance of at least 100 km, the multi-core optical fiber comprising: a plurality of cores each extending along a predetermined axis and arranged on a cross section perpendicular to the predetermined axis; and a cladding region integrally surrounding each of the cores , wherein the plurality of cores rotate around the predetermined axis, and wherein where an average twist rate defined as an average value of absolute values of angles of rotations of the cores per unit length along a longitudinal direction of the multi-core optical fiber (1a; 1b; 1c; 1; 2) is γ (rad/m), the shortest distance between centers of the respective cores is A (m), a group index for a fundamental mode in each of the cores is n g , an in-use bending radius defined as a bending radius in use of the multi-core optical fiber (1a; 1b; 1c; 1; 2) is R (m), the speed of light in vacuum is c (m/s), and the ratio of the circumference of a circle to its diameter is π, the multi-core optical fiber (1a; 1b; 1c; 1; 2) satisfies at least one of first to third conditions and satisfies a fourth condition, the first condition being defined so that Expression : 2 n g Λ cR γπ is not more than 7.91×10 -12 (s/m 1/2 ), the second condition being defined so that in a state in which the multi-core optical fiber (1a; 1b; 1c; 1; 2) is wound on a bobbin with a radius R bobbin (m), Expression : 2 n g Λ cR bobbin γπ is not more than 7.91×10 -12 ×1/R bobbi , (s/m 1/2 ), the third condition being defined so that the shortest distance A is not more than approximately 25×10 -6 m and the average twist rate γ is not less than approximately 4.72 rad/m, when the bending radius of the fiber in use is 0.2m or more, and the fourth condition being defined so that crosstalk between closest adjacent cores is not less than - 15 dB. wherein the plurality of cores rotate around the predetermined axis, and wherein where an average twist rate defined as an average value of absolute values of angles of rotations of the cores per unit length along a longitudinal direction of the multi-core optical fiber (1a; 1b; 1c; 1; 2) is γ (rad/m), the shortest distance between centers of the respective cores is A (m), a group index for a fundamental mode in each of the cores is n g , an in-use bending radius defined as a bending radius in use of the multi-core optical fiber (1a; 1b; 1c; 1; 2) is R (m), the speed of light in vacuum is c (m/s), and the ratio of the circumference of a circle to its diameter is π, the multi-core optical fiber (1a; 1b; 1c; 1; 2) satisfies at least one of first to third conditions and satisfies a fourth condition, the first condition being defined so that Expression : 2 n g Λ cR γπ is not more than 7.91×10 -12 (s/m 1/2 ), the second condition being defined so that in a state in which the multi-core optical fiber (1a; 1b; 1c; 1; 2) is wound on a bobbin with a radius R bobbin (m), Expression : 2 n g Λ cR bobbin γπ is not more than 7.91×10 -12 ×1/R bobbi , (s/m 1/2 ), the third condition being defined so that the shortest distance A is not more than approximately 25×10 -6 m and the average twist rate γ is not less than approximately 4.72 rad/m, when the bending radius of the fiber in use is 0.2m or more, and the fourth condition being defined so that crosstalk between closest adjacent cores is not less than - 15 dB. <EOS>