Multicore fiber

A multicore fiber comprising:

• a plurality of center cores;
• a plurality of low-refractive-index layers each of which encloses outer periphery of one of the center cores;
• and
• a cladding portion that encloses the plurality of low-refractive-index layers, wherein
• maximum refractive index of the center cores is greater than average refractive index of the cladding portion,
• minimum refractive index of the low-refractive-index layers is smaller than average refractive index of the cladding portion,
• among the plurality of center cores, absolute value of difference in relative refractive-index differences of maximum refractive indexes of the center cores with respect to average refractive index of the cladding portion is equal to or smaller than 10%, and absolute value of difference in center core diameters is equal to or smaller than 10%, and
• regarding two low-refractive-index layers from among the plurality of low-refractive-index layers, absolute value of difference in relative refractive-index differences of minimum refractive indexes of the two low-refractive-index layers with respect to average refractive index of the cladding portion is equal to or greater than 10%, or absolute value of difference in ratios of outer diameters with respect to the center core diameters is equal to or greater than 10%.