Crystalline silicate is often found in comets as an 11.3-micron resonant emission 
feature, and may be used for probing the early history of the solar nebula. Because 
the formation of the crystalline silicate grains requires high temperature, they 
are thought to be born from amorphous silicate at the inner region of the early 
solar nebula, and then transported toward the outer region where comets were born. 
This transportation can produce the difference in the fraction of crystalline 
silicate in the cometary dust between two dynamical types of comets, Oort-cloud 
comets (OCs) and Ecliptic comets (ECs), due to the different heliocentric distances 
of their birth places. To verify whether a diffusive transportation was effective 
in the early solar nebula or not, a quantitative study of crystalline silicate in 
comets is crucial. Actually, no clear crystalline silicate feature has been reported 
for ECs until Subaru observations. Subaru/COMICS is a powerful tool for this objective. 
Since 2003, we have carried out continuous mid-infrared spectroscopic observations 
of comets with Subaru/COMICS, and more than ten comets have been observed thus far. 
Especially the Subaru/COMICS has succeeded in detecting the clear crystalline 
silicate feature in several EC spectra, such as 9P/Tempel at the "Deep Impact" event 
and 17P/Holmes just after the explosive outburst phenomenon. From these observations, 
it is suggested that OCs and ECs with different dynamic characteristics may have a 
similar origin. However, much more observational samples are necessary to clarify 
this issue. This observational program provides important data-sets for understanding 
what had occured in the early solar nebula.
orption line does not show large change in strength over 3-5 years.
This may suggest that lower-ionization Fe~II absorbers have
clumpy/filamentary structures with larger volume density,
floating in higher-ionization C~IV absorbers with lower volume density.

arried out through collaborations