  We report on the galaxy density environment around a high-z radio galaxy
(HzRG) at z=4.72, HSC J083913.17+011308.1 (HSC J0839+0113), probed using an
r-dropout Lyman break galaxy (LBG) sample from the Hyper Suprime-Cam Subaru
Strategic Program data. We find that HSC J0839+0113 resides in the outskirt of
an overdense region identified by the r-dropout galaxies at a 4.7 sigma
significance level. The projected distance between HSC J0839+0113 and the peak
position of the overdense region is 0.4 physical Mpc which is shorter than the
typical protocluster radius in this epoch. According to the extended Press
Schechter and the light cone models, the HSC J0839+0113-hosted overdense region
is expected to evolve into a halo > 10^14 Msun at z=0 with a high probability
of >80 %. These findings suggest that HSC J0839+0113 is associated with a
protocluster. The HSC J0839+0113 rich-system is the most overdense region of
LBGs among the known protoclusters with LBGs in the same cosmic epoch.
