  We investigate the extinction curves of young galaxies in which dust is
supplied from Type II supernovae (SNe II) and/or pair instability supernovae
(PISNe). Since at high redshift (z>5), low-mass stars cannot be dominant
sources for dust grains, SNe II and PISNe, whose progenitors are massive stars
with short lifetimes, should govern the dust production. Here, we theoretically
investigate the extinction curves of dust produced by SNe II and PISNe, taking
into account reverse shock destruction induced by collision with ambient
interstellar medium. We find that the extinction curve is sensitive to the
ambient gas density around a SN, since the efficiency of reverse shock
destruction strongly depends on it. The destruction is particularly efficient
for small-sized grains, leading to a flat extinction curve in the optical and
ultraviolet wavelengths. Such a large ambient density as n_H > 1 cm^{-3}
produces too flat an extinction curve to be consistent with the observed
extinction curve for SDSS J104845.05+463718.3 at z=6.2. Although the extinction
curve is highly sensitive to the ambient density, the hypothesis that the dust
is predominantly formed by SNe at z~6 is still allowed by the current
observational constraints. For further quantification, the ambient density
should be obtained by some other methods. Finally we also discuss the
importance of our results for observations of high-z galaxies, stressing a
possibility of flat extinction curves.
