Multicore optical fiber

A multicore optical fiber comprising:

• a glass optical fiber including a plurality of cores each extending along a central axis, a marker extending along the plurality of cores, and a cladding surrounding the plurality of cores and the marker;
• and
• a resin coating provided on an outer peripheral surface of the glass optical fiber,
• wherein on a cross section of the multicore optical fiber orthogonal to the central axis, centers of the plurality of cores and a center of the marker constitute a plane figure having a rotational symmetry of order 1 with respect to a center of the cross section,
• on the cross section, the marker is disposed to be located on a circumference of a circle having a center coinciding with the center of the cross section and having the circumference passing through the centers of the plurality of cores,
• a refractive index of the marker is lower than a refractive index of the cladding, and
• a refractive index of each of the plurality of cores is higher than the refractive index of the cladding.