A method of a posteriori error estimation is proposed in this paper where the spatial and time discretization errors are estimated separately. There arise two kinds of discretization errors for dynamic finite element analyses, one is for space domain, the other is for time domain. A posteriori spatial error estimation for dynamic plate bending problems in each time step has been proposed by authors, while a posteriori time error estimation is under our consideration in this paper. A posteriori error estimation both for spatial and time discretization error is developed for dynamic plate bending problems. Two numerical examples are solved in order to demonstrate the efficiency and validity of the proposed method.