The present embodiment relates to a multi-core optical fiber and others having an excellent transmission capacity per unit cross-sectional area and spectral efficiency and being easy to manufacture. The multi-core optical fiber has a plurality of cores, a cladding, and a coating and has an appropriately-set neighboring core pitch, cable cutoff wavelength, confinement index of light into each core, cladding outer diameter, and neighboring intercore power coupling coefficient, and a relationship among a minimum of Formula Λ/(rclad−OCTmin), a maximum of the Formula, and a core count is present under a predetermined relationship.