  We report on the confirmation that the candidate transits observed for the
star EPIC 211525389 are due to a short-period Neptune-sized planet. The host
star, located in K2 campaign field 5, is a metal-rich ([Fe/H] = 0.26$\pm$0.05)
G-dwarf (T_eff = 5430$\pm$70 K and log g = 4.48$\pm$0.09), based on
observations with the High Dispersion Spectrograph (HDS) on the Subaru 8.2m
telescope. High-spatial resolution AO imaging with HiCIAO on the Subaru
telescope excludes faint companions near the host star, and the false positive
probability of this target is found to be <$10^{-6}$ using the open source
vespa code. A joint analysis of transit light curves from K2 and additional
ground-based multi-color transit photometry with MuSCAT on the Okayama 1.88m
telescope gives the orbital period of P = 8.266902$\pm$0.000070 days and
consistent transit depths of $R_p/R_\star \sim 0.035$ or $(R_p/R_\star)^2 \sim
0.0012$. The transit depth corresponds to a planetary radius of $R_p =
3.59_{-0.39}^{+0.44} R_{\oplus}$, indicating that EPIC 211525389 b is a
short-period Neptune-sized planet. Radial velocities of the host star, obtained
with the Subaru HDS, lead to a 3\sigma\ upper limit of 90 $M_{\oplus} (0.00027
M_{\odot})$ on the mass of EPIC 211525389 b, confirming its planetary nature.
We expect this planet, newly named K2-105 b, to be the subject of future
studies to characterize its mass, atmosphere, spin-orbit (mis)alignment, as
well as investigate the possibility of additional planets in the system.
