  We construct a new algorithm for estimating the star formation rate (SFR) of
galaxies from their infrared (IR) luminosity by developing the theory of the IR
emission from a dusty H II region.
  The derived formula is ${\rm SFR}/(\MO {\rm yr^{-1}})=\{3.3\times
10^{-10}(1-\eta)/
  (0.4-0.2f+0.6 \epsilon)\}(L{\rm ^{obs}_{IR}}/\LO)$, where $f$ is the fraction
of ionizing photons absorbed by hydrogen,
  $\epsilon$ is the efficiency of dust absorption for nonionizing photons from
OB stars, and $\eta$ is the cirrus fraction of observed IR luminosity.
  The previous conversion formulae of SFR from the IR luminosity is applicable
to only the case where the observed IR luminosity is nearly equal to the
bolometric luminosity (starburst galaxies etc.), except for some empirical
formulae.
  On the other hand, our theoretical SFR is applicable to galaxies even with a
moderate star formation activity.
  That is, our simple and convenient formula is significantly useful for
estimating the SFR of various morphologies and types of galaxies --- from early
elliptical to late spiral and irregular galaxies, or from active starburst to
quiescent galaxies --- as far as they have neither an abnormal dust-to-gas
ratio nor an evident active galactic nucleus.
