  The Ultra Luminous X-ray Sources (ULXs) are unique in exhibiting moderately
bright X-ray luminosities, $L_{\rm x} \sim 10^{38-40} {\rm erg~s^{-1}}$, and
relatively high blackbody temperatures, $\Tin \sim 1.0-2.0 {\rm keV}$.
  From the constraint that $L_{\rm x}$ cannot exceed the Eddington luminosity,
$L_{\rm E}$, we require relatively high black-hole masses, $M\sim 10-100
M_\odot$, however, for such large masses the standard disk theory predicts
lower blackbody temperatures, $\Tin < 1.0$ keV. To understand a cause of this
puzzling fact, we carefully calculate the accretion flow structure shining at
$\sim L_{\rm E}$, fully taking into account the advective energy transport in
the optically thick regime and the transonic nature of the flow. Our
calculations show that at high accretion rate ($\dot M \ga 30~L_{\rm E}/c^2$)
an apparently compact region with a size of $\Rin \simeq (1-3)\rg$ (with $\rg$
being Schwarzschild radius) is shining with a blackbody temperature of $\Tin
\simeq 1.8 (M/10M_\odot)^{-1/4}$ keV even for the case of a non-rotating black
hole. Further, $\Rin$ decreases as $\dot M$ increases, on the contrary to the
canonical belief that the inner edge of the disk is fixed at the radius of the
marginally stable last circular orbit. Accordingly, the loci of a constant
black-hole mass on the "H-R diagram" (representing the relation between $L_{\rm
x}$ and $\Tin$ both on the logarithmic scales) are not straight but bent
towards the lower $M$ direction in the frame of the standard-disk relation.
