A multi-core optical fiber comprising: at least one coupled-core group constituted by a plurality of cores; and a common cladding surrounding the coupled-core group , wherein, where an average value of a curvature of the multi-core optical fiber in a state in which the multi-core optical fiber is embedded in a cable, along a longitudinal direction of the multi-core optical fiber, is denoted by C avg [m -1 ], a virtual curvature of the multi-core optical fiber in a linear state is denoted by C f [m -1 ], a mode coupling coefficient between adjacent cores among the plurality of cores constituting the coupled-core group is denoted by κ [m -1 ], an average value of propagation constants of the adjacent cores in the coupled-core group is denoted by β [m -1 ], and a core center interval between the adjacent cores in the coupled-core group is denoted by A [m], the multi-core optical fiber has a core structure and a core arrangement satisfying following first to third conditions: the first condition defined such that the core interval A between the adjacent cores in the coupled-core group is set such that the mode coupling coefficient κ between the adjacent cores at a wavelength of 1550 nm is 1.6 × 10 2 [m -1 ] or less; the second condition defined such that κ/(βΛC avg ) or κ/(βΛC f ) becomes 0.1 or less in a wavelength range of 1530 to 1625 nm when C avg is set to 0.01 to 2 [m -1 ] or 0.1 to 1 [m -1 ]; and the third condition defined such that κ becomes 2.6 × 10 0 [m -1 ] or more, or, κ/(βΛC avg ) or κ/(βΛC f ) becomes 0.01 or more in the wavelength range of 1530 to 1625 nm. at least one coupled-core group constituted by a plurality of cores; and a common cladding surrounding the coupled-core group , wherein, where an average value of a curvature of the multi-core optical fiber in a state in which the multi-core optical fiber is embedded in a cable, along a longitudinal direction of the multi-core optical fiber, is denoted by C avg [m -1 ], a virtual curvature of the multi-core optical fiber in a linear state is denoted by C f [m -1 ], a mode coupling coefficient between adjacent cores among the plurality of cores constituting the coupled-core group is denoted by κ [m -1 ], an average value of propagation constants of the adjacent cores in the coupled-core group is denoted by β [m -1 ], and a core center interval between the adjacent cores in the coupled-core group is denoted by A [m], the multi-core optical fiber has a core structure and a core arrangement satisfying following first to third conditions: the first condition defined such that the core interval A between the adjacent cores in the coupled-core group is set such that the mode coupling coefficient κ between the adjacent cores at a wavelength of 1550 nm is 1.6 × 10 2 [m -1 ] or less; the second condition defined such that κ/(βΛC avg ) or κ/(βΛC f ) becomes 0.1 or less in a wavelength range of 1530 to 1625 nm when C avg is set to 0.01 to 2 [m -1 ] or 0.1 to 1 [m -1 ]; and the third condition defined such that κ becomes 2.6 × 10 0 [m -1 ] or more, or, κ/(βΛC avg ) or κ/(βΛC f ) becomes 0.01 or more in the wavelength range of 1530 to 1625 nm. the first condition defined such that the core interval A between the adjacent cores in the coupled-core group is set such that the mode coupling coefficient κ between the adjacent cores at a wavelength of 1550 nm is 1.6 × 10 2 [m -1 ] or less; the second condition defined such that κ/(βΛC avg ) or κ/(βΛC f ) becomes 0.1 or less in a wavelength range of 1530 to 1625 nm when C avg is set to 0.01 to 2 [m -1 ] or 0.1 to 1 [m -1 ]; and the third condition defined such that κ becomes 2.6 × 10 0 [m -1 ] or more, or, κ/(βΛC avg ) or κ/(βΛC f ) becomes 0.01 or more in the wavelength range of 1530 to 1625 nm. <EOS>