  We re-examine the effect of Lyman continuum ($\lambda \leq 912$ \AA)
extinction (LCE) by dust in H {\sc ii} regions in detail and discuss how it
affects the estimation of the global star formation rate (SFR) of galaxies. To
clarify the first issue, we establish two independent methods for estimating a
parameter of LCE ($f$), which is defined as the fraction of Lyman continuum
photons contributing to hydrogen ionization in an H {\sc ii} region. One of
those methods determines $f$ from the set of Lyman continuum flux, electron
density and metallicity. In the framework of this method, as the metallicity
and/or the Lyman photon flux increase, $f$ is found to decrease. The other
method determines $f$ from the ratio of infrared flux to Lyman continuum flux.
Importantly, we show that $f \la 0.5$ via both methods in many H {\sc ii}
regions of the Galaxy. Thus, it establishes that dust in such H {\sc ii}
regions absorbs significant amount of Lyman continuum photons directly. To
examine the second issue, we approximate $f$ to a function of only the
dust-to-gas mass ratio (i.e., metallicity), assuming a parameter fit for the
Galactic H {\sc ii} regions. We find that a characteristic $\hat{f}$, which is
defined as $f$ averaged over a galaxy-wide scale, is 0.3 for the nearby spiral
galaxies. This relatively small $\hat{f}$ indicates that a typical increment
factor due to LCE for estimating the global SFR ($1/\hat{f}$) is large ($\sim
3$) for the nearby spiral galaxies. Therefore, we conclude that the effect of
LCE is not negligible relative to other uncertainties of estimating the SFR of
galaxies.
