In this research, ETO (Engineer-to-Order) production with MTS (Make-to-Stock) parts is focused on. The research aims to enable the sales staff to quickly identify customer requirements, propose required product functional specifications and delivery dates while considering efficient utilization of parts inventory. To realize this aim, a sales support system that presents product functional specifications to customers in following two phases is proposed. (1)Customized design phase: to enable quick proposition of specifications based on the customer requirements, the relationship among customer requirements and product specifications are systematized, and a basic product functional specification determination method is proposed. (2) Order reservation and selection phase: to enable efficient use of parts inventory and maximize orders, order receiving seats are developed beforehand by applying the concept of production seat booking system . Basic product functional specification obtained from ( 1) is used as input to enable convenient selection of specifications for the manufacturer. A case study of machine tools is tested, and the effects are confirmed on both increasing order receive rates and reducing parts inventory.