A multi-core optical fiber (1; 1A; 1B) comprising: a plurality of cores each extending along a central axis and comprised of silica-based glass; a common cladding covering each of the plurality of cores and comprised of silica glass; and a coating provided on an outer peripheral surface of the common cladding and comprised of a material different from silica glass, wherein in a cross section of the multi-core optical fiber perpendicular to the central axis, the plurality of cores are arranged in an equilateral-triangular lattice pattern and a neighboring core pitch A [μm] being a distance between centers of neighboring cores is constant, wherein a cable cutoff wavelength λ cc [μm] of each of the plurality of cores is not more than 1.53 μm, wherein when a theoretical cutoff wavelength is represented by λ cth [μm] and a mode field diameter by MFD [μm], MFD/(λ cc) 0.657 as a confinement index of light into each of the plurality of cores is not more than 8.2 at the wavelength of 1.55 μm, or, MFD/(λ cth) 0.720 as a confinement index of light into each of the plurality of cores is not more than 6.9 at the wavelength of 1.55 μm, wherein an outer diameter 2r clad [μm] of the common cladding is from 115 μm to 250 μm, wherein when OCT represents a shortest distance from a center of a core located at an outermost periphery out of the plurality of cores, to an outer peripheral surface of the common cladding, OCT 0.01dB/km [μm] by which a transmission loss increase in the coating theoretically becomes 0.01 dB/km and OCT 0.001dB/km [μm] by which the transmission loss increase in the coating theoretically becomes 0.001 dB/km, are values obtained by Formula or Formula below using MFD [μm] at the wavelength of 1.625 μm, { OCT 0.01 ⁢ dB / km = 1.625 − 7.78 × 10 − 2 MFD λ cc 0.869 2 + 3.02 MFD λ cc 0.869 OCT 0.001 ⁢ dB / km = 1.625 − 8.48 × 10 − 2 MFD λ cc 0.869 2 + 3.31 MFD λ cc 0.869 { OCT 0.01 ⁢ dB / km = 1.625 − 2.90 × 10 − 2 MFD λ cth 0.703 2 + 2.81 MFD λ cth 0.703 OCT 0.001 ⁢ dB / km = 1.625 − 3.06 × 10 − 2 MFD λ cth 0.703 2 + 3.08 MFD λ cth 0.703 wherein when permissible minimum OCT is represented by OCT min, the OCT min is a value from OCT 0.01dB/km to OCT 0.001dB/km and actual OCT is not less than OCT min, wherein with a definition of a parameter represented by a formula of Λ/(r clad -OCT min) which is a ratio of the neighboring core pitch A to a core-arrangeable radius of the common cladding, a relationship of [a core count: a minimum of the parameter: a maximum of the parameter] is one of [19: 0.450: 0.500], [27: 0.378: 0.397], [31: 0.351: 0.378], [37: 0.315: 0.333], [42: 0.297: 0.311], [48: 0.278: 0.285], [55: 0.255: 0.277], [63: 0.240: 0.248], [69: 0.229: 0.240], [73: 0.222: 0.229], [85: 0.203: 0.218], [102: 0.189: 0.195], [109: 0.182: 0.189], [121: 0.173: 0.180], and [151: 0.154: 0.160] characterised in that when the multi-core optical fiber is bent in a radius of curvature R in the range of 0.3 m to 3 m over the entire length thereof, both of a neighboring intercore power coupling coefficient and a neighboring identical intercore power coupling coefficient are not more than 2.3×10 -6 /km at the wavelength of 1625 nm and at least either one of the neighboring intercore power coupling coefficient and the neighboring identical intercore power coupling coefficient is not less than 3.6×10 -9 /km at the wavelength of 1530 nm.