The efficiency of event selection is an important parameter in the measurement of r lepton mass. In our study we use so-called semi-experimental method in which we combine M.C. events and experimental data. With this method the efficiency of selecting ττ → eμ(v) is obtained as (14.5±0.7±0.8)%, and various relevant corrections are discussed. The result is consistent with that derived from two-parameter fitting of τ-lepton mass.