The mass of the τ lepton has been measured in the decay mode τ→3πντ using a pseudomass technique. The result obtained from 414fb-1 of data collected with the Belle detector is Mτ=[1776.61±0. 13(stat)±0.35(sys)]MeV/c2. The upper limit on the relative mass difference between positive and negative τ leptons is |Mτ+-Mτ-|/ Mτ<2.8×10-4 at 90% confidence level. © 2007 The American Physical Society.