Multicore fiber

A multicore fiber comprising:

• 7 or more cores;
• and
• a clad covering outer circumferences of the respective cores and formed in a circular shape on a cross-section thereof, wherein
• diameters of the adjacent cores differ from one another,
• each of the cores performs single-mode propagation at a transmission wavelength,
• a relative refractive index difference of each of the cores against the clad is less than 1.4%,
• a center-to-center distance between the adjacent cores is less than 50 μm,
• in a case where the transmission wavelength of each of the cores is λ, the center-to-center distance between the adjacent cores is , a mode field diameter at the transmission wavelength 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
• a distance between the outer circumference of the core farthest from a center and an outer circumference of the clad is 2.5 or higher times as long as the mode field diameter of each of the cores.