One of the reasons that reverberation degrades speech intelligibility is the effect of overlap-masking, in which segments of an acoustic signal are affected by reverberation components of previous segments [Bolt et al., 1949]. To reduce the overlap-masking, Arai et al. suppressed steady-state portions having more energy, but which are less crucial for speech perception, and confirmed promising results for improving speech intelligibility [Arai et al., 2002]. Our goal is to provide a pre-processing filter for each auditorium. To explore the relationship between the effect of a pre-processing filter and reverberation conditions, we conducted a perceptual test with steady-state suppression under various reverberation conditions. The results showed that processed stimuli performed better than unprocessed ones and clear improvements were observed for reverberation conditions of 0.8 - 1.0s. We certified that steady-state suppression was an effective pre-processing method for improving speech intelligibility under reverberant conditions and proved the effect of overlap-masking.