<p>A multicore optical fiber (1A) comprises: cores (10 - 16); a visible marker (20); and common cladding (30) surrounding these cores (10 - 16) and the visible marker (20). The cores (10 - 16) and the visible marker (20) extend in the fiber axis direction. The refractive index of the visible marker (20) is different from the refractive index of the cladding (30). The cores (10 - 16) are arranged to have six-fold rotational symmetry and line symmetry on the cross-section orthogonal to the fiber axis. The visible marker (20) is arranged to break the symmetries. The visible marker (20) is arranged on the inner side of a minimum diameter circle containing the cores (10 - 16) with a fiber core as the center.</p>
