The stress intensity factor in the elasto-dynamics has been defined from the stress near cracks and notches so that the accuracy of numerical computations might be necessary. The present report trys to define this factor from the variation of time mean of the potential energy of the crack surface as the same way as in the elasto-statics. It is found that this potential energy corresponds to Lagrangean, that is, the difference of the strain energy and kinetic energy, and that the amplitude of diffracted wave. Applying the theory, especially for a crack growing geometrically similarly, a simple formula are derived. Numerical results shows a slight difference from the usual value which has obtained from the stress near ends of a crack. The usual definition has been derived from a semi-infinite crack, so that it might not be correct for a finite crack and a dynamical case.
