To perform an extended test of the Lorentz structure of the charged weak interaction, a study of five-body leptonic decays is ongoing with data sample which contains about 0.91 × 109 τ+τ− pairs, collected at Belle. The Standard Model predicts that the Lorentz structure has a V-A structure and this can be tested through the measurement of Michel parameters. In our study we try to give tighter constraints to Michel parameters through the measurement of branching fraction of five-body leptonic decays of tau. With an embedded formalism of total differential decay width which has recently been published, Monte Carlo event generator has been developed within TAUOLA program. In this paper, we finally report our preliminary result of systematic uncertainties of the branching fractions.