  From the Hamiltonian connecting the inside and outside of an Fabry-Perot
cavity, which is derived from the Maxwell boundary conditions at a mirror of
the cavity, a master equation of a non-Lindblad form is derived when the cavity
embeds matters, although we can transform it to the Lindblad form by performing
the rotating-wave approximation to the connecting Hamiltonian. We calculate
absorption spectra by these Lindblad and non-Lindblad master equations and also
by the Maxwell boundary conditions in the framework of the classical
electrodynamics, which we consider the most reliable approach. We found that,
compared to the Lindblad master equation, the absorption spectra by the
non-Lindblad one agree better with those by the Maxwell boundary conditions.
Although the discrepancy is highlighted only in the ultra-strong light-matter
interaction regime with a relatively large broadening, the master equation of
the non-Lindblad form is preferable rather than of the Lindblad one for
pursuing the consistency with the classical electrodynamics.
