In 1980's, a new type of X-ray sources, which have a blackbody-like spectrum with a
temperature of 20-100 eV, was discovered by the Einstein observatory in the Large
Magellanic Cloud. They were defined as a new class, called Supersoft X-ray Sources
(SSS), which was reinforced by subsequent additional discoveries with ROSAT. SSS are
white dwarf (WD) bi- nary systems and their X-ray luminosities are neaar the Eddington
limit due to steady nuclear burning on the WD surface. In addition to such persistent
SSS, SSS spectra have frequently been observed in Classical Novae (CNe) and Recurrent
Novae (RNe) during the later phase of their outbursts (called SSS phase).

Two X-ray observatories, onboard the XMM-Newton and Chandra, were launched in
1999. Their X-ray grating spectrometers with a high spectral resolution have unveiled
ex- treme complexity of the SSS spectra; SSS spectra contain not only a blackbody-like
contin- uum, but also numerous emission and/or absorption lines and absorption
edges. Blackbody- like continuum, absorption lines, and absorption edges are considered
to originate from the optically-thick WD atmosphere, whereas emission lines are from the
optically-thin plasma surrounding the central WD called the accretion disk corona (ADC).

SSS spectra have a wide range of spectral variety. A systematic study of persistent SSS
and novae during SSS phase revealed that SSS can be classified into two types by its
spectral features; absorption line dominated (SSa) or emission line dominated (SSe).
Both SSa and SSe have a mixed spectrum of the direct emission from the WD atmosphere and
the reprocessed emission in the ADC. However, the direct emission of the latter may be
obscured by the intervening matter such as flared disk. As a result, SSe spectra are
dominated by reprocessed emission with numerous emission line features. This
interpretation can explain the observational results that the SSe spectra are often
observed in the SSS with high inclination angles of >70 deg , whereas the SSa spectra
are observed in the SSS with lower or unknown inclination angles.

A model that can describe both the SSa and SSe spectra in a unified manner has not been
constructed to date. In particular, contrast to the almost established direct emission
from the WD atmosphere, understanding of the reprocessed emission lags behind
significantly.

In this thesis, we made an attempt to make such a model. We first constructed a model
representing the SSe spectra based on the Monte Carlo approach. This approach is suited
for our study investigating the physical process of the photons in the ADC. Then, we
combined our reprocessed model and the NLTE model for the WD atmosphere, and
successfully explained SSS spectra of both SSe and SSa types. Finally, we constructed a
unified picture explaining both SSa and SSe spectra by considering that the effect of
the direct component is obscured or not.