  Recent observations have revealed that red, optically--passive spiral
galaxies with little or no optical emission lines, harbour significant amounts
of dust-obscured star formation. We propose that these observational results
can be explained if the spatial distributions of the cold gas and star-forming
regions in these spiral galaxies are significantly more compact than those in
blue star-forming spirals. Our numerical simulations show that if the sizes of
star-forming regions in spiral galaxies with disk sizes of R_d are ~ 0.3R_d,
such galaxies appear to have lower star formation rates as well as higher
degrees of dust extinction. This is mainly because star formation in these
spirals occurs only in the inner regions where both the gas densities and
metallicities are higher, and hence the dust extinction is also significantly
higher. We discuss whether star formation occurring preferentially in the inner
regions of spirals is closely associated with the stripping of halo and disk
gas via some sort of environmental effect. We suggest that the "outside-in
truncation of star formation" is the key to a better understanding of
apparently optically--passive spirals with dusty star-forming regions.
