The present invention relates to a multicore optical fiber having a structure for suppressing core-to-core crosstalk. The multicore optical fiber (100A) comprises a plurality of cores extending along a predetermined axis while being arranged like a hexagonal lattice on a cross section perpendicular to the axis and a cladding region (120) integrally surrounding the plurality of cores. All of core portions, each constituting at least a part of the associated one of the plurality of cores, have substantially the same structure.