An optical fiber 10 is one in which at a mean zero dispersion wavelength lambada<0> in an overall length, an absolute value of the fourth order dispersion beta<4> of propagation constant with respect to frequency is not more than 5*10<-56>s<4>/m and a fluctuation of the zero dispersion wavelength along the longitudinal direction is not more than +-0.6 nm in the overall length. The optical fiber 10 comprises at least a center core part 11 having a maximum refractive index N<1> and an outside diameter 2a, a depressed part 12 having a minimum refractive index N<2> and an outside diameter 2b, and a cladding part 13 having a maximum refractive index N<3>. The refractive indices of the respective parts satisfy the relation of ''N<1>>N<3> > N<2>''. With respect to the refractive index of the cladding part, a relative index difference of the center core part is defined as delta<+> and a relative index difference of the depressed part as delta<->; then a difference ''delta<+>-delta<->'' is not less than 2.2%, and a ratio Ra (= 2a/2b) of the outside diameters of the center core part and the depressed part is in the range of 0.2 to 0.7.