When a sound with a gap is presented as a deviant stimulus in a repetition of a continuous sound as a standard stimulus, the gap triggers MMN (Desjardins, 1999). In experiment 1, we examined whether the latency and the amplitude of the MMN varies with the duration difference. In experiment 2, we examined whether the similar MMN is elicited when the sound with a gap was presented as the standard and the continuous sound was presented as the deviant. The results of experiment 1 showed that both the latency and the amplitude were constant in all conditions of the gap duration. On the other hand, the results of experiment 2 showed that the latency and the amplitude became longer and smaller respectively as the gap duration became shorter. Asymmetry of the MMN found in these results suggests that some deviant features are easy to be detected and others are difficult for the deviant detection system, and that the system uses different strategies to detect each deviant feature.