To provide a dispersion shift optical fiber which is large in a mode field diameter and is small in a dispersion slope in a use wavelength band.

This optical fiber is formed by covering the outer peripheral side of a center core 1, covering the outer peripheral side of a first side core 2 with a second side core 3, covering the outer peripheral side of the second side core 3 with an inside clad 4 and covering the outer peripheral side of the inside clad 4 with an outside clad 5. When the specific refractive index difference of the center core 1 with respect to the outside clad 5 is defined as Δ1, the specific refractive index difference of the first side core 2 with respect to the outside clad 5 as Δ2, the specific refractive index difference of the second side core 3 with respect to the outside clad 5 as Δ3 and the specific refractive index difference of the inside clad 4 with respect to the outside clad 5 as Δ4, Δ1>Δ3>Δ4>Δ2 is set and 0.75%≤Δ1≤0.85%, -0.15%≤Δ2≤0, 0.3%≤Δ3≤0.5%, -0.2%≤Δ4≤0 are set.