The use of the constant intensity approximation in the theory of fifth-harmonic generation in dissipative media, with an account of reverse harmonic reaction on the exciting wave phase, has been developed. The optimal values of lengths for all three crystals when the conversion efficiency to the fifth harmonic is maximum have been obtained. It has been shown that, when the optimal parameter values of the problem are chosen, the high efficiency of the fifth-harmonic cascade generator can be reached. Under phase-matching conditions in nondissipative media, the maximum efficiency is approximately 40%.