Concerning the study of capsizing of a ship and the evaluation of nonlinear wave loads acting on a ship, the numerical simulation method is necessary since the nonharmonic forces are concerned in the problem. To deal with the practical problems, it is recognized that the Strip Theory is useful and in this paper, based on the Strip Theory, the equation of motion described in the time domain with a convolution integral is developed. Numerical results obtained by using of it are compared with those of the second order linear differential equation with constant coefficients.