  The $O(a)$ improved Wilson quark action on the anisotropic lattice is
investigated. We carry out numerical simulations in the quenched approximation
at three values of lattice spacing ($a_{\sigma}^{-1}=1$--2 GeV) with the
anisotropy $\xi=a_{\sigma}/a_{\tau}=4$, where $a_{\sigma}$ and $a_{\tau}$ are
the spatial and the temporal lattice spacings, respectively. The bare
anisotropy $\gamma_F$ in the quark field action is numerically tuned by the
dispersion relation of mesons so that the renormalized fermionic anisotropy
coincides with that of gauge field. This calibration of bare anisotropy is
performed to the level of 1 % statistical accuracy in the quark mass region
below the charm quark mass. The systematic uncertainty in the calibration is
estimated by comparing the results from different types of dispersion
relations, which results in 3 % on our coarsest lattice and tends to vanish in
the continuum limit. In the chiral limit, there is an additional systematic
uncertainty of 1 % from the chiral extrapolation.
  Taking the central value $\gamma_F=\gamma_F^*$ from the result of the
calibration, we compute the light hadron spectrum. Our hadron spectrum is
consistent with the result by UKQCD Collaboration on the isotropic lattice. We
also study the response of the hadron spectrum to the change of anisotropic
parameter, $\gamma_F \to \gamma_F^* + \delta\gamma_F$. We find that the change
of $\gamma_F$ by 2 % induces a change of 1 % in the spectrum for physical quark
masses. Thus the systematic uncertainty on the anisotropic lattice, as well as
the statistical one, is under control.
