  The present paper describes the analysis of multiple RXTE/PCA data of the
black hole binary with superluminal jet, XTE J1550-564, acquired during its
1999--2000 outburst. The X-ray spectra show features typical of the high/soft
spectral state, and can approximately be described by an optically thick disk
spectrum plus a power-law tail. Three distinct spectral regimes, named standard
regime, anomalous regime, and apparently standard regime, have been found from
the entire set of the observed spectra. When the X-ray luminosity is well below
6E38 erg/s (assuming a distance of 5 kpc), XTE J1550-564 resides in the
standard regime, where the soft spectral component dominates the power-law
component and the observed disk inner radius is kept constant. When the
luminosity exceeds the critical luminosity, the apparently standard regime is
realized, where luminosity of the optically thick disk rises less steeply with
the temperature, and the spectral shape is moderately distorted from that of
the standard accretion disk. In this regime, radial temperature gradient of the
disk has been found to be flatter than that of the standard accretion disk. The
results of the apparently standard regime are suggestive of a slim disk (e.g.,
Abramowicz et al. 1988, Watarai et al. 2000) which is a solution predicted
under high mass accretion rate. In the intermediate anomalous regime, the
spectrum becomes much harder, and the disk inner radius derived using a simple
disk model spectrum apparently varies significantly with time. These properties
can be explained as a result of significant thermal inverse Comptonization of
the disk photons, as was found from GRO J1655-40 in its anomalous regime by
Kubota, Makishima and Ebisawa (2001).
