Excitonic effects of optical gain in quantum wells have been studied theoretically in a three-band model. Taking into account an additional localized level in the energy gap, we obtain the optical gain in terms of the transition between a localized state and a continuous subband state with small carrier concentration. Simultaneously, excitonic absorption also occurs near the band-edge transition. It is shown that the optical gain is strongly enhanced by excitonic effects through the coupling between the two transitions. This enhanced optical gain might show the realization of very-low-threshold current-laser diodes.