When a cylinder floating on the free surface starts an arbitrary oscillation, the flow field around the body should change in complicated manner. This paper deals with a wave making theory of the cylinder at the early stage of the oscillation. An integral equation with two variables, depending on time and location on the section was introduced for obtaining the source density distributions over the immersed surface of the cylinder by using the time dependent Green's function. The kernel function of the integral equation is not suitable to integrate itself numericallly in direct, when the time variables become large. In order to elude the difficalty of the integration for that case, the function was expanded to an appropriate asymptotic series. Then, the numerical solution of the integral equation was obtained. However, it becomes clear that a reasonable solution is hardly obtained, as the integral equation involves an eigen function. Only an approximate solution was obtained in this paper. Further investigation is necessary to introduce an integral equation without eigen function.