In hot rolling process, rolled plate is cooled by water cooling so that plate temperature distribution is uniformed and reaches desired one. Through this process, quality of finished plate can be kept to high level. Conventional water cooling is based on statistical model made of the past data. But, water cooling based on statistical model of the past can't be employed if new specifications of plate products are required. The objective of our reserch is to develop a water cooling method based on mathematical models so that problems for arbitrarily-shaped plates are to be treated. Mathematical nonlinear models of controlled plate temperature distribution are created. Based on the mathematical model, linearized model is obtained by Galerkin's method. Optimal regulator is applied to plate temperature distribution control. The validity of the proposed method was checked through numerical experiments.