  We present the first results of the Subaru/Hyper Suprime-Cam (HSC) survey of
the interacting galaxy system, NGC4631 and NGC4656. From the maps of resolved
stellar populations, we identify 11 dwarf galaxies (including already-known
dwarfs) in the outer region of NGC4631 and the two tidal stellar streams around
NGC4631, named Stream SE and Stream NW, respectively. This paper describes the
fundamental properties of these tidal streams. Based on the tip of red giant
branch method and the Bayesian statistics, we find that StreamSE (7.10 Mpc in
Expected a posteriori, EAP, with the 90% credible intervals of [6.22, 7.29]
Mpc) and StreamNW (7.91 Mpc in EAP with the 90% credible intervals of [6.44,
7.97] Mpc) are located in front of and behind NGC4631, respectively. We also
calculate the metallicity distribution of stellar streams by comparing the
member stars with theoretical isochrones on the color-magnitude diagram. We
find that both streams have the same stellar population based on the Bayesian
model selection method, suggesting that they originated from a tidal
interaction between NGC4631 and a single dwarf satellite. The expected
progenitor has a positively skewed metallicity distribution function with
[M/H]_EAP=-0.92 with the 90% credible intervals of [-1.46, -0.51]. The stellar
mass of the progenitor is estimated as 3.7 x 10e+8 Msun with the 90% credible
intervals of [5.8 x 10e+6, 8.6 x 10e+9] Msun based on the mass-metallicity
relation for Local group dwarf galaxies. This is in good agreement with an
initial stellar mass of the progenitor presumed in the previous N-body
simulation.
