As it is impossible to assume complete rationality in a social dilemma situation, the assumption of bounded rationality is appropriate. Under the bounded rationality, it would be reasonable to assume that one behaves according to heuristic principle. The group identity effect in a social dilemma situation might be very important in order to attain cooperation. The aim of this study was to clarify how to promote cooperative behavior by avoiding the social dilemma situations. The group heuristics was taken into account, and it was explored how the group heuristics promotes a cooperative behavior in social dilemma situations. As a result of a two-person game experiment, the group heuristics was found to play an important role in a social dilemma situation, and enhance cooperative behavior. For the following three cases, the higher cooperation rate was attained at the latter half of the experiment: (1)mutual in-group condition, (2)one-way in-group condition, and (6)one-way unknown condition (in-group). In conclusion, consciousness of in-group membership might help to promote actively mutual cooperation.