  We investigate the upper stellar mass limit set by radiative feedback by the
forming star with various accretion rates and metallicities. To this end, we
numerically solve the structures of both a protostar and its surrounding
accretion envelope assuming a spherical symmetric and steady flow. The optical
depth of the dust cocoon, a dusty part of the accretion envelope, differs among
the direct light from the stellar photosphere and the diffuse light re-emitted
as dust thermal emission. As a result, varying the metallicity qualitatively
changes the way that the radiative feedback suppresses the accretion flow. With
a fixed accretion rate of $10^{-3}M_{\odot} {\rm yr^{-1}}$, the both direct and
diffuse lights jointly operate to prevent the mass accretion at $Z \gtrsim
10^{-1}Z_{\odot}$. At $Z \lesssim 10^{-1}Z_{\odot}$, the diffuse light is no
longer effective, and the direct light solely limits the mass accretion. At $Z
\lesssim 10^{-3}Z_{\odot}$, the HII region formation plays an important role in
terminating the accretion. The resultant upper mass limit increases with
decreasing metallicity, from a few $\times~10~M_\odot$ to $\sim 10^3~M_\odot$
over $Z = 1Z_{\odot} - 10^{-4}~Z_\odot$. We also illustrate how the radiation
spectrum of massive star-forming cores changes with decreasing metallicity.
First, the peak wavelength of the spectrum, which is located around $30 \mu
{\rm m}$ at $1Z_{\odot}$, shifts to $< 3 \mu {\rm m}$ at $Z \lesssim 0.1
Z_\odot$. Second, a characteristic feature at $10 \mu \rm{m}$ due to the
amorphous silicate band appears as a dip at $1Z_{\odot}$, but changes to a bump
at $Z \lesssim 0.1 Z_{\odot}$. Using these spectral signatures, we can search
massive accreting protostars in nearby low-metallicity environments with
up-coming observations.
