  We report on the global structure of the Milky Way (MW) stellar halo up to
its outer boundary based on the analysis of blue-horizontal branch stars
(BHBs). These halo tracers are extracted from the $(g,r,i,z)$ band
multi-photometry in the internal data release of the on-going Hyper Suprime-Cam
Subaru Strategic Program (HSC-SSP) surveyed over $\sim550$~deg$^2$ area. In
order to select most likely BHBs by removing blue straggler stars (BSs) and
other contamination in a statistically significant manner, we have developed
and applied an extensive Bayesian method, instead of the simple color cuts
adopted in our previous work, where each of the template BHBs and non-BHBs
obtained from the available catalogs is represented as a mixture of multiple
Gaussian distributions in the color-color diagrams. We found from the candidate
BHBs in the range of 18.5<g<23.5 mag that the radial density distribution over
a Galactocentric radius of r=36-360 kpc can be approximated as a single
power-law profile with an index of $\alpha=3.74^{+0.21}_{-0.22}$ or a broken
power-law profile with an index of $\alpha_{\rm in}=2.92^{+0.33}_{-0.33}$ at
$r$ below a broken radius of $r_{\rm b}=160^{+18}_{-19}$~kpc and a very steep
slope of $\alpha_{\rm out}=15.0^{+3.7}_{-4.5}$ at $r>r_{\rm b}$. The latter
profile with a prolate shape having an axial ratio of $q=1.72^{+0.44}_{-0.28}$
is most likely and this halo may hold a rather sharp boundary at r=160kpc. The
slopes of the halo density profiles are compared with those from the suite of
hydrodynamical simulations for the formation of stellar halos. This comparison
suggests that the MW stellar halo may consist of the two overlapping
components: the in situ. inner halo as probed by RR Lyrae stars showing a
relatively steep radial density profile and the ex situ. outer halo with a
shallow profile probed by BHBs here, which is made by accretion of small
stellar systems.
