  We present results of optical spectroscopic observations of the mass donor
star in SS 433 with Subaru and Gemini, with an aim to best constrain the mass
of the compact object. Subaru/FOCAS observations were performed on October 6-8
and 10, 2007, covering the orbital phase of phi=0.96-0.26. We first calculate
cross correlation function of these spectra with that of the reference star HD
9233 in the wavelength range of 4740-4840 Angstrom. This region is selected to
avoid 'strong' absorption lines accompanied with contaminating emission
components. The same analysis is applied to archive data of Gemini/GMOS taken
at phi=0.84-0.30 by Hillwig & Gies (2008). From the Subaru and Gemini CCF
results, the amplitude of radial velocity curve of the donor star is determined
to be 58.3+/-3.8 km s-1 with a systemic velocity of 59.2+/-2.5 km s-1. Together
with the radial velocity curve of the compact object, we derive the mass of the
donor star and compact object to be M_O=12.4+/-1.9 M_sun and M_X=4.3+/-0.6
M_sun, respectively. We conclude, however, that these values should be taken as
upper limits. From the analysis of the averaged absorption line profiles of
strong lines and weak lines observed with Subaru, we find evidence for heating
effects from the compact object. Using a simple model, we find that the true
radial velocity amplitude of the donor star could be as low as 40+/-5 km s-1 in
order to produce the observed absorption-line profiles. Taking into account the
heating of the donor star may lower the derived masses to M_O=10.4 +2.3/-1.9
M_sun and M_X=2.5 +0.7/-0.6 M_sun. Our final constraint, 1.9 M_sun< M_X <4.9
M_sun, indicates that the compact object in SS 433 is most likely a low mass
black hole, although the possibility of a massive neutron star cannot be firmly
excluded.
