Along with expanding the understanding of the human 'social brain', a new challenge for neuroscience is to elucidate the nature of individual differences in social competence. Here we report a neural index of gender difference in empathy-related processing in a complex social situation. Using electroencephalography, we measured the neural activity of perceptions to one's own and another's monetary gain or loss, while individuals played a 'competitive' two-person gambling game, in which one's monetary gain resulted in the other's loss. The medial-frontal negativity (MFN), a component within 300 ms latency reflecting an emotional categorization of the event, showed a significant gender difference in perceiving an opponent's, but not a self-performed outcome. When females perceive the opponent's outcomes, the MFN was elicited, indicating that another's loss was categorized as negative, even though it resulted in a benefit to them. On the contrary, the males did not elicit discernable MFN to the opponent's outcomes. Together with the fact that the affect score has a negative linear correlation with the MFN, this indicated that the MFN was sensitive to socio-emotional processing. These results suggest that individual differences in complex social behavior result from rapid neural activity in response to external stimuli.