Reflectance spectra and magnetizations of a diluted magnetic semiconductor Cd0.989Co0.011Se have been studied in high magnetic fields up to 25T. The A-exciton line appeared at 1.840 eV, which split in magnetic fields parallel to the c-axis. The splitting energy increased proportionally to the magnetization up to approximately 7T, while it increased linearly to the magnetic field in the fields higher than 12T, where the magnetization is saturated. The diamagnetic shift of the exciton, which is quadratic as a function of the magnetic field, was remarkably observed above 12T. We estimated that the sp-d exchange constant N0(α-β)=2.52 (eV), the effective g-value 3gh-ge=1.7 and the constant of the diamagnetic shift σ=6.8×10-6(eV/T2).