隨著醫療技術的進步和生活環境的改善，人類的平均壽命也隨之延長，面臨需要更多退休金的準備。為了解決退休金給付缺口和財富移轉等問題，確定提撥制逐漸成為世界退休金制度的主要趨勢。確定提撥退休金制度當中有一種員工持股信託退休金計畫，由於可促成勞資一體向心力，有許多資本額大、員工人數眾多的企業採用。該制度存在退休金資產過度集中化的問題，本文提出一個保證契約，讓計畫參與者在退休時有權將員工持股退休金計劃內股票轉換成特定的分散性投資組合。此保證契約之理論費用可以看成是員工持股退休金計劃風險分散的隱含成本。本文發現風險分散隱含成本與公司股票風險和分散性投資組合風險呈現正關係，和公司股票報酬與分散性投資組合報酬的相關係數、分散性投資組合的股利率呈反關係。其中風險分散隱含成本對於分散性投資組合的股利發放率的敏感度特別地高，且距離退休年齡愈長愈敏感，分散性投資組合股利發放率的參數對於風險分散隱含成本的計算是相當重要的。 As a result of advances in medical technology and improvements in the living environment,the average human lifespan has increased, creating a situation where people need to save more money for retirement than would have been the case in the past. In response to the shortfall in pension funding and the related wealth transfer issues, there has been a gradual global trend towards adoption of the defined-contribution system, as opposed to the defined-benefit system. One of the various types of defined-contribution pension plans is the employee stock ownership trust (ESOT). The ESOT system has been adopted by many business enterprises with high capitalization and a large number of employees, principally because it is believed to strengthen unity of purpose and cohesion between business owners and employees. However, one problem with the ESOT system is that pension assets are highly concentrated in company stock. This study proposes a type of guarantee agreement whereby, employees participating in an ESOT scheme would receive at least the value of a well-diversified portfolio at the time of retirement. The theoretical expenses associated with this guarantee agreement can be regarded as implied costs of diversification for company stock in the ESOT plan. The study finds that implied costs of diversification are positively related to the volatilities of company stock and the diversified portfolio, while they are negatively related to the return on company stock, the correlation between the company stock and the diversified portfolio, and the dividend paid by the diversified portfolio.