A dispersion compensating fiber (DCF), which is an amplification medium of an S-band discrete Raman amplifier (RA), has a trapezoid core, an inner cladding surrounding the trapezoid core; and an outer cladding surrounding the inner cladding. A ring is disposed between the inner cladding and the outer cladding. The refractive index plotted across the diameter of the trapezoid core as a function of distance is substantially trapezoidal in shape. The difference in refractive index between the trapezoid core and the outer cladding is 1.2 to 1.6%; between the inner cladding and the outer cladding is 0.4 to 0.8%; and between the ring and the outer cladding is 0.2 to 0.6%. The thickness of the ring is 0.8 to 1.2 times the radius of the trapezoid core, and the thickness of the inner cladding is 1 to 3 times the radius of the trapezoid core.