The single sourcing policy, in which an automobile manufacturer purchases identical or similar parts from one supplier, has an advantage of scale economy. Meanwhile, multiple sourcing policy, which allows procuring similar parts from multiple suppliers, has benefits of dispersing risks and promoting competition among suppliers. This paper analyzes the procurement policies by presenting a model of the Japanese automobile parts trade. It concludes that maturity of technology involved should be taken into account besides above-mentioned factors which have traditionally been recognized. For parts produced using evolving technologies, the single sourcing enhances purchaser’s benefits because of the scale economy in learning process. In the meantime, multiple sourcing is more beneficial to the purchaser if the parts are based on mature technologies. In either policy, if the technology involved is evolving, motivating suppliers by returning a great part of cost reduction as a reward to them may eventually increase profit for the purchaser. The conclusion supports the situation where the number of suppliers is being cut down as the trend of modularization and system deliveries of parts progresses in the auto parts industry, and suggests that returning part of benefits to parts suppliers may be encouraged from the viewpoint of auto manufacturers’ own interest.