This study aimed to analyze the decision-making process during a one-shot prisoner’s dilemma game and test the social exchange heuristic hypothesis. To this end, we used an eye-tracking measure to examine whether the cooperator subjectively transformed the one-shot prisoner’s dilemma game into the assurance game. According to the group heuristic model and the social exchange hypothesis, cooperation in a one-shot prisoner’s dilemma game is more pronounced toward in-group members, and the mechanism is thought to arise from the subjective transformation; furthermore, cooperation in a one-shot prisoner’s dilemma game, especially toward in-group members, should be based on quick decision making. To test these hypotheses, we carefully analyzed the decision-making process of participants during one-shot prisoner’s dilemma games. Our results suggest that decision time is faster for those who cooperate with in-group members than for those who do not cooperate. In addition, cooperators, regardless of who they cooperate with, generally pay more attention to the outcome of mutual cooperation than non-cooperators. These findings support the social exchange heuristic hypothesis.