  Analysis was made of the multiple XTE/PCA data on the promised black hole
candidate with superluminal jet, GRO J1655-40, acquired during its 1996--1997
outburst. The X-ray spectra can be adequately described by the sum of an
optically thick disk spectrum and a power-law. When the estimated 1--100 keV
power-law luminosity exceeds 1E37 erg/s (assuming a distance of 3.2 kpc), the
inner disk radius and the maximum color temperature derived from a simple
accretion disk model (a multi-color disk model) vary significantly with time.
These results reconfirm the previous report by Sobczak et al. (1999). In this
strong power-law state (once called ``very high state''), the disk luminosity
decreases with temperature, in contradiction to the prediction of the standard
Shakura-Sunyaev model. In the same state, the intensity of the power-law
component correlates negatively with that of the disk component, and positively
with the power-law photon index, suggesting that the strong power-law is simply
the missing optically thick disk emission. One possible explanation for this
behavior is inverse-Compton scattering in the disk. By re-fitting the same data
incorporating a disk Comptonization, the inner radius and temperature of the
underlying disk are found to become more constant. These results provide one of
the first observational confirmations of the scenario of disk Comptonization in
the strong power-law state. This strong power-law state seems to appear when
color temperature of the disk exceeds the certain threshold, 1.2 -- 1.3 keV.
