We have observed that anomalously large optical gain emerges at the spectral position of the lowest heavy-hole exciton transition in Cd1-xMnxTe/Cd1-yMgyTe quantum wells (x = 0.02, y = 0.33) with the resonant excitation 1condition by 200 fs laser pulse at 7T and 5K. The gain is enhanced by increasing the applied field. The maximum gain value with the injection energy of 6.0 mu J/cm(2)/pulse reaches 3.5 x 10(4)cm(-1) which corresponds to 13 of the linear absorption maximum of the exciton transition at 7 T. Further, the gain grows rapidly along with the injection of pump pulse and relaxes with the decay time less than 1 ps. These experimental results suggest that the gain is induced by the optical polarization of excitons which is created by the optical field of pump pulse and also that gain relaxatation is connected to the dephasing time of the exciton coherence. Further, we have observed that the exciton absorption band splits into three bands before the gain appears, indicating the splitting is induced by the optical Stark effect. The results are discussed in terms of the dressed-atom model.