We report the results of mid- to far-infrared spectroscopic
observations of Galactic star-forming regions with ISO, Spitzer, and
AKARI. Owing to the high sensitivity of the IRS onboard Spitzer, we
detected [Si II] 35 μm, [Fe II] 26 μm, and [Fe III] 23 μm lines widely
in low-density star-forming regions, and derived gas-phase Si and Fe
abundances as 3–100% and <22%, respectively. With the FTS
onboard AKARI, we obtained the spatial distribution of
the [O III] 88 μm emission in two star-forming regions.