本文嘗試以部份均衡模型爲基礎，利用Shy and Stenbacka (2003)對委外成本的假設，探討不同市場結構下，最終財廠商的委外決策。模型推導以漸進式進行，首先討論當最終財市場結構爲獨占下，最終財廠商的委外決策；其次開放市場競爭程度，使最終財市場結構由獨占轉爲寡占，廠商家數由一擴增爲二，且兩廠商在最終財市場進行同質產品的數量競爭，則最終財廠商的委外決策是否改變。藉由模型推導得到：當最終財市場結構爲獨占，且代工廠與委外廠商存在著相同的固定成本下，委外行爲並不會成立。因爲在上下游皆獨占的結構中，上游廠商所得到的收益並不足以負擔固定成本的支出，而形成即便是下游廠商願意委外，上游廠商卻無力代工的窘境。然而當最終財市場開放，競爭加劇時。一方面最終財廠商因市場競爭，所得到的市場份額會相對縮小，收益自然減少，造成負擔固定成本支出的能力下降，進而提高委外生產的意願。另一方面中間投入廠商藉由獲得最終財兩廠商對中間投入需求的提高，可以得到的收益因而增加，負擔固定成本支出的能力也較爲提高，而產生當最終財兩廠商因負擔不起設廠生產的固定成本轉而尋求委外代工時，中間投入廠商亦願爲代工，使得雙委外情況得以成立。本文主要發現爲即使中間投入廠商不具有生產成本的優勢，只要最終財廠商面對更爲競爭的環境，都可能使得委外生產情形發生，亦即市場競爭程度爲廠商進行委外代工的誘因與動機之一。

In this paper, we build a partial equilibrium model to explore a downstream firm's outsourcing decision in the final goods market by utilizing the assumption of outsourcing costs in Shy and Stenbacka (2003). First, we discuss a monopolistic downstream firm's decision of outsourcing in the final good market. Then we extend the model to a duopolistic downstream market structure with two competing firms to examine whether the outsourcing decisions change in such a competitive environment. It is found that, when the final good market structure is monopoly, no outsourcing occurs under that both the upstream firm (i.e., the outsourcing service provider) and the outsourcing firm incur the same fixed cost. This is because the revenue the upstream firm earns is not sufficient to cover the fixed cost of setting up a manufacturing factory such that outsourcing is unprofitable for the upstream firm even when the downstream firm has an incentive to outsource. Nevertheless, outsourcing does occur if the downstream market appears to be competitive. In this case, there exist two effects: one is that, due to the market competition, the duopolistic downstream firms' market shares and their revenues decrease, thereby lowering their ability to burden the fixed costs of setting up a manufacturing factory; the other is that the increasing demand for the intermediary goods from the downstream firms raises the upstream firm's incentive to accept the outsourcing contract. This article main discovery ever the upstream firm does not have the advantages of product cost, as long as the final goods firm facing more competes to the market, possibly causes outside the committee the production situation occurrence, i.e. the market competition degree carries on outside the committee for the firm the generation of labor cause and motives one.