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 1998 - 1999 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, which we call the "standard regime,'' the "anomalous regime,'' and the "apparently standard regime,'' have been found from the entire set of the observed spectra. When the X-ray luminosity is well below similar to 6 x 10(38) ergs s(-1) (assuming a distance of 5 kpc), XTE J1550 - 564 resides in the standard regime, in which 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, in which the 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, the 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 suggest a slim disk, which is a solution predicted for a high mass accretion rate. In the intermediate anomalous regime ( or very high state in the literature), 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 and coworkers.
