  Some supernovae (SNe) are powered by collision of the SN ejecta with a dense
circumstellar matter (CSM). Their emission spectra show characteristic line
shapes of combined broad emission and narrow P-Cyg lines, which should closely
relate to the CSM structure and the mass-loss mechanism that creates the dense
CSM. We quantitatively investigate the relationship between the line shape and
the CSM structure by Monte Carlo radiative transfer simulations, considering
two representative cases of dense CSM formed by steady and eruptive mass loss.
Comparing the H$\alpha$ emission between the two cases, we find that a narrow
P-Cyg line appears in the eruptive case while it does not appear in the steady
case, due to the difference in the velocity gradient in the dense CSM. We also
reproduce the blue-shifted photon excess observed in some Type IIn SNe, which
is formed by photon transport across the shock wave and find the relationship
between the velocity of the shocked matter and the amount of the blue shift of
the photon excess. We conclude that the presence or absence of narrow P-Cyg
lines can distinguish the mass loss mechanism, and suggest high-resolution
spectroscopic observations with $\lambda/ \Delta \lambda \gtrsim 10^4$ after
the light curve peak for applying this diagnostic method.
