A multicore fiber 1 includes: a small diameter portion 33 in which a propagation constant of light of an x1-th order LP mode of the first core 11 (here, x1 is an integer of “2” or more and x or less, x is an integer of “2” or more) and a propagation constant of light of a y1-th order LP mode of the second core 12 (here, y1 is an integer of “1” or more and y or less other than x1, y is an integer of “1” or more) coincide with each other and a large diameter portion in which a propagation constant of light of each LP mode of the first core 11 and a propagation constant of light of each LP mode of the second core 12 are configured not to coincide with each other are arranged.