© 2016, Japanese Society for Artificial Intelligence. All rights reserved. The purpose answer the question whether an artificial agent can build or not an in-group relation with a human. To answer this, we created a three-way discussion setting between a participant and two artificial agents: One agent took the same opinion as a participant while the other did the opposite. This resulted in a feeling of group identity between the participant and the first agent, i.e. the same side. This feeling of group identity was shown to be detected using error-related negativity (ERN), a component of an event-related potential that accompanies errors in speeded performance. We found that amplitude of ERN was bigger for the same side agent’s failure than for the other side’s. We also found ERN difference correlated to an empathy ability detected by a questionnaire. From these results, we can say that a person can form an in-group relation between an artificial agent, which can be detected by ERN.