  We have determined interstellar extinction law toward the Galactic center
(GC) at the wavelength from 1.2 to 8.0 micron, using point sources detected in
the IRSF/SIRIUS near-infrared survey and those in the 2MASS and
Spitzer/IRAC/GLIMPSE II catalogs. The central region |l| < 3deg and |b| < 1deg
has been surveyed in the J, H and Ks bands with the IRSF telescope and the
SIRIUS camera whose filters are similar to the Mauna Kea Observatories (MKO)
near-infrared photometric system. Combined with the GLIMPSE II point source
catalog, we made Ks versus (Ks - lambda) color-magnitude diagrams where lambda
= 3.6, 4.5, 5.8, and 8.0 micron. The Ks magnitudes of bulge red clump stars and
the (Ks - lambda) colors of red giant branches are used as a tracer of the
reddening vector in the color-magnitude diagrams. From these magnitudes and
colors, we have obtained the ratios of total to selective extinction
A(Ks)/E(Ks-lambda) for the four IRAC bands. Combined with A(lambda)/A(Ks) for
the J and H bands derived by Nishiyama et al., we obtain
A(J):A(H):A(Ks):A([3.6]):A([4.5]):A([5.8]):A([8.0])=3.02:1.73:1:0.50:0.39:0.36:0.43
for the line of sight toward the GC. This confirms the flattening of the
extinction curve at lambda > 3 micron from a simple extrapolation of the
power-law extinction at shorter wavelengths, in accordance with recent studies.
The extinction law in the 2MASS JHKs bands has also been calculated, and a good
agreement with that in the MKO system is found. In nearby molecular clouds and
diffuse interstellar medium, the lack of reliable measurements of the total to
selective extinction ratios hampers unambiguous determination of the extinction
law; however, observational results toward these lines of sight cannot be
reconciled with a single extinction law.
