  We discuss the absorption spectrum of dirty s-wave superconductors in the
nonequilibrium steady state under a homogeneous and monochromatic microwave. In
this state, there exists a finite density of states at lower energies than the
superconducting gap, and the distribution function is also defined in this
energy region. Because of the reduction of the distribution function below the
superconducting gap, the absorption by thermally excited quasiparticles is
suppressed in the nonequilibrium state compared with that in the equilibrium
case. This is in contrast to the cases of the gap function and the absorption
by direct excitation, to which the variation of the distribution function above
the superconducting gap mainly contributes. It is also shown that amplitude
modes are generated by frequency mixing of the pump wave and the probe wave,
and two peaks appear in the absorption spectrum.
