The interaction between a company and customers has been important to build brand loyalty which affects the decision making of customers. On the other hand, the interaction between customers is now considered to become more important because of the popularization of the Internet. For this background. We analyze the effect of the interaction between customers on brand loyalty by agent-based approach. We build a simulation model with product companies and consumers. While the companies promote their own products. The consumers exchange information by words of mouse and decide to buy. We analyze the effects by comparing the simulation results between with and without interactions.