Today, there are several markets in cyber space where companies trade electronically due to the development of Information Technology. On the other hand, the most important thing in trades is negotiation. So, in order to support current business practices as well as new ones on the Internet, electronic commerce systems need an ability to negotiate. In this paper, proposed is a method by which a seller can be supported by an agent which plays a role in negotiation process among small and medium companies especially injection mold manufacturer. If the manufacturing capacity cannot afford to produce all orders, the manufacturer may want to extend due dates and the buyers may want to discount prices. The negotiation agent discussed in this paper cooperates with the schedule agent to get due-date information, and performs a role in one (seller)-to-many (buyer) negotiation processes.