  We study internal extinction of late-type galaxies in the Sloan Digital Sky
Survey. We find that the degree of internal extinction depends on both the
concentration index c and K_s-band absolute magnitude M_K. We give simple
fitting functions for internal extinction. In particular, we present analytic
formulae giving the extinction-corrected magnitudes from the observed optical
parameters. For example, the extinction-corrected r-band absolute magnitude can
be obtained by M_{r,0}=-20.77 +(-1+\sqrt{1+4\Delta
(M_{r,obs}+20.77+4.93\Delta)})/2\Delta, where \Delta =
0.236\{1.35(c-2.48)^2-1.14\}\log(a/b), c=R_{90}/R_{50} is the the concentration
index, and a/b is the isophotal axis ratio of the 25 mag/arcsec^2 isophote in
the i-band. The 1\sigma error in M_{r,0} is 0.21{\rm log}(a/b). The late-type
galaxies with very different inclinations are found to trace almost the same
sequence in the (u-r)-$M_r$ diagram when our prescriptions for extinction
correction are applied. We also find that (u-r) color can be a third
independent parameter that determines the degree of internal extinction.
