  The lifetime of the $\tau$-lepton is measured using the process
$e^+e^-\rightarrow\tau^+\tau^-$, where both $\tau$-leptons decay to
$3\pi\nu_\tau$. The result for the mean lifetime, based on
$711\,\mathrm{fb}^{-1}$ of data collected with the Belle detector at the
$\Upsilon(4S)$ resonance and $60\,\mathrm{MeV}$ below, is $\tau = (290.17 \pm
0.53(\mathrm{stat.}) \pm 0.33(\mathrm{syst.}))\cdot10^{-15}\,\mathrm{s}$. The
first measurement of the lifetime difference between $\tau^+$ and $\tau^-$ is
performed. The upper limit on the relative lifetime difference between positive
and negative $\tau$-leptons is $|\Delta\tau| / \tau < 7.0 \times 10^{-3}$ at
90% CL.
