Constraints on the dark mass distribution surrounding Sgr A*: simple
  $\chi^2$ analysis for the redshift of photons from orbiting stars
  Sagittarius A* (Sgr A*) is the central supermassive black hole with the mass
$\sim 4\times 10^6 M_{\odot}$ in the Milky Way and stars are orbiting around
it. In May 2018, one of the nearest stars to Sgr A* named S0-2/S2 experienced
the pericenter passage. The redshift of photons from S0-2 had varied from
$4000\>{\rm km\>s^{-1}}$ to $-2000\>{\rm km\>s^{-1}}$ during the pericenter
passage, which is within $0.5\>{\rm yr}$. In this paper, we show that this
steep variation of the redshift gives a strong constraint on a dark mass
distribution inside the orbit of S0-2. By applying a simple $\chi^2$ analysis
to the observed redshift, we can easily distinguish between the two models, the
point mass model and the point mass plus an extended mass model without the
best-fitting parameter search. Our redshift data during the pericenter passage
in 2018 with Subaru/IRCS bound the amount of the extended mass inside the orbit
of S0-2 less than $0.5\,\%$ ($\sim 2 \times 10^4 M_\odot$) of the mass of Sgr
A*. This constraint obtained by our simple analysis is comparable to previous
works with the best-fitting parameter search to the motion of S0-2 including
the effect of the extended mass. We consider both the power-law and the Plummer
models for the dark mass distribution model, but the significant difference
between these results is not found.
