The present invention relates to an optical fiber for light pulse expansion in which the ratio (.beta..sub.3/.beta..sub.2) of the third derivative .beta..sub.3 to the second derivative .beta..sub.2 is negative, the absolute value thereof is large, and the absolute value of the second derivative .beta..sub.2 is also large. Such an optical fiber comprises at least a central core portion having a maximum refractive index N.sub.1 and an outer diameter 2a, a depressed portion, provided on the outer periphery of the central core portion, having a minimum refractive index N.sub.2 and an outer diameter 2b, and a cladding portion, provided on the outer periphery of the depressed portion, having a maximum refractive index N.sub.3. The respective maximum refractive indices of the central core portion, the depressed portion and the cladding portion satisfy the relationship "N.sub.1>N.sub.3>N.sub.2".