The purpose of the present study was to examine how the existence of multiple Prisoner's Dilemma (PD) relationships influence the development of a network system among participants and behavior in each PD relationship. The experiment consisted of two conditions. In one condition (network condition), subjects were required to make a two-step decision. The first step was to choose a game partner from the four-member group. The second step was to determine how much one should cooperate with his or her partner in the PD relationships. Repeated trials included both decision steps. In the second condition (Two-person PD condition), subject could not choose their partner, and were required to repeatedly play the PD game with the same partner. Result showed that: 1) In the network condition, a commitment between partners developed spontaneously; 2) Rate of cooperation increased in members who had established a commitment, compared to the trial when one had yet to establish such a commitment; and 3) the cooperative relationship was more firmly maintained in the network condition than in the two-person PD condition.