Manufacturing method of multicore fiber preform, multicore fiber preform, and multicore fiber

A multicore fiber comprising:

• n pieces of first core regions in a polygonal shape that are arranged to have a center of gravity at points P21 to P2n, and that has a first core portion and a first cladding portion formed on an outer circumference of the first core portion, by setting the points P21 to P2n at positions, distances of which are X21 to X2n, respectively, from a point a2, on straight lines dividing an end surface into n pieces to have an equal central angle about the point a2 of the end surface such that a plurality of overlap portions in which adjacent polygons overlap each other at least in a part thereof are formed;
• a second core region in a polygonal shape that is arranged about the point a1, that has a second core portion and a second cladding portion formed on an outer circumference of the second core portion, and that includes all of the overlap portions;
• and
• a cladding region that is formed on an outer circumferences of the first core region and the second core region, wherein
• abutting surfaces that are flat surfaces abutting on each other are formed in portions on the outer circumferences of the first core region and the second core region, and
• another cladding region having a softening temperature lower than the first cladding portion and the cladding region is arranged in place of the second core region.