Abstract: The present invention provides a dispersion compensating optical fiber, connected to a transmission single-mode optical fiber having zero dispersion in a 1.3-.mu.m wavelength band, for efficiently compensating for the dispersion of a light signal of a 1.55-.mu.m wavelength band. The refractive index configuration is such that the refractive index profile is a W type and that the dispersion value is a high negative value of not more than -80. The ratio Ra (Ra=a/b) of the diameter a of a core 1 and the diameter b of a side core 2 is set within a range of not less than 0.33 and not more than 0.37. The ratio DPS(SMF)/DPS(DCF) of a value DPS(SMF) obtained by dividing the dispersion value of the transmission single-mode optical fiber in the 1.55-.mu.m wavelength band by the dispersion slope of the transmission single-mode optical fiber in the 1.55-.mu.m wavelength to a value DPS(DCF) obtained by dividing the dispersion value of the dispersion compensating optical fiber in the 1.55-.mu.