  We present an Ha spectral observation of a Balmer-dominated shock on the
eastern side of Tycho's supernova remnant using the Subaru Telescope. Utilizing
the High Dispersion Spectrograph (HDS), we measure the spatial variation of the
line profile between preshock and postshock gas. Our observation clearly shows
a broadening and centroid shift of the narrow-component postshock Ha line
relative to the Ha emission from the preshock gas. The observation supports the
existence of a thin precursor where gas is heated and accelerated ahead of the
shock. Furthermore, the spatial profile of the emission ahead of the Balmer
filament shows a gradual gradient in the Ha intensity and line width ahead of
the shock. We propose that this region (~10^16 cm) is likely to be the
spatially resolved precursor. The line width increases from ~30 up to ~45 km/s,
and its central velocity shows a redshift of ~5 km/s across the shock front.
The characteristics of the precursor are consistent with a cosmic-ray
precursor, although the possibility of a fast neutral precursor is not ruled
out.
