Recently, the importance of sustainable energy using renewable resources has been recognized. As one of the sustainable energy resources, ocean thermal energy conversion (OTEC) is important for the development of a next-generation energy source. This paper proposes a method to construct a state-space model for an OTEC plant using the Rankine cycle. The state-space model is constructed based on a linear approximation of a simple dynamic model. The flow rates of warm seawater, cold seawater, and working fluid are considered as inputs to the model, and the power output is selected as the output of the model. The state-space model, described as a time-invariant system, is derived numerically. The constructed model is evaluated through numerical simulations of the step response; the power generation control is verified using proportional integral controllers, and the advantage as a state-space model is verified using a linear quadratic regulator. The simulation results demonstrate the effectiveness and limitation of the proposed model. Because the simple dynamic model to be approximated is constructed based on the physical laws, various kinds of state-space models corresponding to the physical parameters can be obtained easily.