對於當事人間財產損益變動之觀察，是論斷不當得利關係之起點。通說就受有利益或損害之判斷係以財產總額為觀察客體，因有一定事實，致財產總額較諸無此事實當時之財產總額為多或為少，即是受有利益或損害。筆者先說明學說就認定財產損益方法上欠妥當之處後，提出以「淨損益」概念作為觀察財產損益之方法，認為對於財產總額增減之觀察，不宜只專注於如標的物或價金等「有形」財產之增減，應同時將債權債務之發生或消滅等「無形」財產之增減一併列入觀察，且所謂財產之增減應以「一定事實」前後作為觀察基準點，倘造成當事人財產損益變動之一定事實不只一項時，具備因果關係（關連性）之各一定事實所造成之財產變動即可合一觀察而得出淨損益，但不具因果關係者，則否。透過淨損益概念之操作，不僅可使不當得利之損益認定更貼近國人計算財產增減之通念，更可於無須更動現有法體系與法價值之前提下，順利地判斷出「誰」受有「何種淨利益（或淨損害）」，再透過一定事實之連結，即可正確地指出不當得利關係之當事人。本文雖輔以指標性不當得利事件初步驗證「淨損益」概念之可行性，惟其實用性與可接受性，仍待更多學說與實務之觀察與檢驗。

Our Civil Code § 179 has ruled that three elements must be established to prove unjust enrichment, namely, benefit and loss, the causation in-between, and no legitimate explanatory basis. Among them, the recognition of the loss and benefit plays the crucial role and is the starting point in unjust enrichment actions, especially in multiple party payment litigations. Unfortunately, legal theorists and learned judges have overlooked some essence in the recognition of the benefit and loss, which conduces to complexity and confusion herein. This article, at the first place, advocates the concept of the “net” benefit and loss, which elaborates the overall assessment of property difference as a consequence of a certain intervening event, tangible or intangible assets all included. Especially highlighted is the importance that the emergence and elimination of creditor’s right and debt should not be ignored when calculating the benefit and loss. And when multiple events intervene, the individual observation on the net benefit and loss can be integrated into a comprehensive one, only if these events exhibit causation in-between. This article also makes special emphasis on the crucial application of this proposed “net” concept in various typical and classical unjust enrichment events and some litigation, in order to examine preliminarily its feasibility and effectiveness. The “net” benefit and loss concept conforms well to the contemporary property concept generally adapted by plebeians, moreover its application transgress neither rules nor regulations of civil law in force. Nevertheless, it proves itself unsophiscated and efficacious in the recognition of “who” bears net benefit and “who” suffers the net loss, and might be helpful in the unjust enrichment judgement.