Action integrals of the periodic orbits of non-integrable (chaotic) two-dimensio nal system of Henon-Heiles were analyzed to show simple linear relations. This e nables us to obtain by extrapolation all the action integrals of any periodic or bits and at any energies from very few arbitrary action integrals. Based on this property, a very simple and easy semiclassical quantization algorithm by numeri cal arithmetics was proposed for low_energy excitations which are classically ch aotic. The result is good agreement with that obtained by the exact quantal met hod.