  Using the Subaru 8.2m Telescope with an IRCS Echelle spectrograph, we
obtained high-resolution (R=10,000) near-infrared (1.01-1.38 \mu m) spectra of
images A and B of the gravitationally lensed QSO B1422+231 (z=3.628) consisting
of four known lensed images. We detected MgII absorption lines at z=3.54, which
show a large variance of column densities (~ 0.3 dex) and velocities (~ 10
km/s) between the sightlines A and B with a projected separation of only
8.4h_{70}^{-1} pc at the redshift. This is the smallest spatial structure of
the high-z gas clouds ever detected after Rauch et al. found a 20-pc scale
structure for the same z=3.54 absorption system using optical spectra of images
A and C. The observed systematic variances imply that the system is an
expanding shell as originally suggested by Rauch et al. By combining the data
for three sightlines, we managed to constrain the radius and expansion velocity
of the shell (~ 50-100 pc, 130 km/s), concluding that the shell is truly a
supernova remnant (SNR) rather than other types of shell objects, such as a
giant HII region. We also detected strong FeII absorption lines for this
system, but with much broader Doppler width than that of \alpha-element lines.
We suggest that this FeII absorption line originates in a localized FeII-rich
gas cloud that is not completely mixed with plowed ambient interstellar gas
clouds showing other \alpha-element low-ion absorption lines. Along with the Fe
richness, we conclude that the SNR is produced by an SNIa explosion.
