  X-ray continuum spectra of super-Eddington accretion flow are studied by
means of Monte Carlo radiative transfer simulations based on the radiation
hydrodynamic simulation data, in which both of thermal and bulk Compton
scatterings are taken into account. We compare the calculated spectra of
accretion flow around black holes with masses of $M_{\rm BH} = 10, 10^2, 10^3$,
and $10^4 M_\odot$ for a fixed mass injection rate (from the computational
boundary at $10^3 r_{\rm s}$) of $10^3 L_{\rm Edd}/c^2$ (with $r_{\rm s}$,
$L_{\rm Edd}$, and $c$ being the Schwarzschild radius, the Eddington
luminosity, and the speed of light, respectively). The soft X-ray spectra
exhibit mass dependence in accordance with the standard-disk relation; the
maximum surface temperature is scaled as $T \propto M_{\rm BH}^{-1/4}$. The
spectra in the hard X-ray bands, by contrast, look quite similar among
different models, if we normalize the radiation luminosity by $M_{\rm BH}$.
This reflects that the hard component is created by thermal and bulk Compton
scattering of soft photons originating from an accretion flow in the
over-heated and/or funnel regions, the temperatures of which have no mass
dependence. The hard X-ray spectra can be reproduced by a Wien spectrum with
temperature of $T\sim 3$ keV accompanied by a hard excess at photon energy
above several keV. The excess spectrum can be well fitted with a power law with
a photon index of $\Gamma \sim 3$. This feature is in good agreement with that
of the recent NuSTAR observations of ULX (Ultra-Luminous X-ray sources).
