This study is a case study of multi-stakeholders involved in sesame production and sale on the island of Kikaijima, which is the largest area of sesame production in Japan. This study found that the transaction structure and forms adopted by actual purchasers’ fierce competition for procurement are likely to lead producers to opportunistic behavior. Subsequently, it causes damage to the growth of the production area. Using in-depth interviews, this research revealed firstly that in Kikaijima, there is fierce competition over the procurement of the rare domestic sesame among the current purchasers. Secondly, the competition that relies solely on price negotiation has resulted in a market environment that induces producers’ opportunistic behavior. Thirdly, the producers’ opportunistic behavior hinders efforts to improve quality and expand the supply needed for the production area’s growth. Lastly, there is an urgent need for producers’ organization and a marketing standard covering all stakeholders in a regional marketing system.