Liability drives insurers' businesses. This paper examines the structural model approach of credit risk for the valuation of insurance liabilities and insurers' equity, and considers a stochastic process for liability. Grosen and Jørgensen's (2002) study presents the current approach taken by insurers; however, the model's structure is very simple, and its liability structure in particular has a deterministic time function. In contrast, we analyze a model that analytically evaluates an insurer's liability with the stochastic process. Furthermore, we analyze the model's default option originally presented by Myers and Read (2001).