The purpose of this study was to examine the role of trust in relationships between temporary partners. In this study, we predicted that trust would play a "signaling" role in promoting mutual cooperation, even in relationships with unfixed or temporary partners. To examine this prediction, we conducted an experiment using two different games. We used the repeated PD/D (prisoner's dilemma with choice of dependence) game, which can measure trusting behavior independently from cooperation. Also, we used the ordinary PD game in which there is no option for trust. Seventy participants were assigned to either the PD/D condition or the ordinary PD condition. In both conditions, players interacted with a randomly matched partner in each trial. The results reveal that the cooperation rate in the PD/D game was significantly higher than that in the PD game. Such a finding indicates that trust serves as a signal of the player's intention, which in turn, promotes mutual cooperation. However, in a similar experiment in which players interacted with the same partner, Matsuda & Yamagishi (2001) found a much lower cooperation rate in the PD/D condition than what was found in this experiment. Therefore, we conclude that the role of trust in non-fixed relationships has only a limited effect for promoting mutual cooperation.