The multicore fiber comprises 7 or more cores, wherein diameters of the adjacent cores differ from one another, wherein each of the cores performs single-mode propagation, wherein a relative refractive index difference of each of the cores is less than 1.4%, wherein a distance between the adjacent cores is less than 50 μm, wherein, in a case where a transmission wavelength of each of the cores is λ, the distance between the adjacent cores is , a mode field diameter of each of the cores is MFD, and a theoretical cutoff wavelength of each of the cores is λc, (/MFD)·(2λc/(λc+λ))≧3.95 is satisfied, and wherein a distance between the outer circumference of the coreand an outer circumference of the clad is 2.5 or higher times as long as the mode field diameter of each of the cores.