  The melting transition in the hard-disk system is considered. Non-equilibrium
relaxation analysis of the six-fold bond-orientational order parameter has been
carried out. The critical point between the hexatic and the fluid phase is
determined on the basis of the dynamic scaling hypothesis. The value of the
critical exponent $\eta$ is determined from the fluctuation of the order
parameter at the criticality as $\eta = 0.25(2)$ which is consistent with the
prediction by the Kosterlitz-Thouless-Halperin-Nelson-Young theory.
