A multi-core optical fiber includes four cores arranged in a shape along a longitudinal direction, each of the four cores having a step-index type refractive index distribution with a radius, and a cladding region having a lower refractive index than that of each core and a diameter of 125±1 μm and provided on an outer portion of each core. An absolute value of a relative refractive index difference between each core and the cladding region is Δ. The four cores are arranged so that a relationship between a minimum distance from the center of each core to an outer periphery of the cladding region, a minimum value A of spacing between the cores, and the MFD at a set wavelength satisfies a Formula C1, and the radius of each core and the relative refractive index difference Δ between the core and the cladding region are set.