The present invention relates to a multicore optical fiber having a structure for effectively inhibiting polarization mode dispersion from increasing, and the multicore optical fiber comprises a plurality of multicore units and a cladding region integrally covering the plurality of multicore units while separating the multicore units from each other. Each of the plurality of multicore units includes a plurality of core regions arranged such as to construct a predetermined core arrangement structure on a cross section orthogonal to an axis. The core arrangement structure of each multicore unit on the cross section has such a rotational symmetry as to coincide with the unrotated core arrangement structure at least three times while rotating by 360° about a center of the multicore unit, thereby reducing the structural asymmetry of each multicore unit. This lowers the structural birefringence in each multicore unit, thereby inhibiting the polarization mode dispersion from increasing in the multicore optical fiber.