The present invention relates to a multi-core optical fiber including a plurality of cores, in each of which an effective area at the wavelength of 1550 nm, a transmission loss at the wavelength of 1550 nm, a chromatic dispersion at the wavelength of 1550 nm, a cable cutoff wavelength, and a bending loss in a bending radius of 30 mm at the wavelength of 1625 nm are set so as to increase a transmission capacity in each core in a state in which a difference of the transmission loss at the wavelength of 1550 nm between different cores is controlled to at most 0.02 dB/km or less.