  A recently discovered young, high-velocity giant star J01020100-7122208 is a
good candidate of hypervelocity star ejected from the Galactic center, although
it has a bound orbit. If we assume that this star was ejected from the Galactic
center, it can be used to constrain the Galactic potential, because the
deviation of its orbit from a purely radial orbit informs us of the torque that
this star has received after its ejection. Based on this assumption, we
estimate the flattening of the dark matter halo of the Milky Way by using the
Gaia DR2 data and the circular velocity data from Eilers et al. (2019). Our
Bayesian analysis shows that the orbit of J01020100-7122208 favors a prolate
dark matter halo within $\sim$ 10 kpc from the Galactic center. The posterior
distribution of the density flattening $q$ shows a broad distribution at $q
\gtrsim1$ and peaks at $q \simeq 1.5$. Also, 98.5\% of the posterior
distribution is located at $q>1$, highly disfavoring an oblate halo.
