  We report the analysis of additional multiband photometry and spectroscopy
and new adaptive optics (AO) imaging of the nearby planetary microlensing event
TCP J05074264+2447555 (Kojima-1), which was discovered toward the Galactic
anticenter in 2017 (Nucita et al.). We confirm the planetary nature of the
light-curve anomaly around the peak while finding no additional planetary
feature in this event. We also confirm the presence of apparent blending flux
and the absence of significant parallax signal reported in the literature. The
AO image reveals no contaminating sources, making it most likely that the
blending flux comes from the lens star. The measured multiband lens flux,
combined with a constraint from the microlensing model, allows us to narrow
down the previously unresolved mass and distance of the lens system. We find
that the primary lens is a dwarf on the K/M boundary (0.581 \pm 0.033 M_sun)
located at 505 \pm 47 pc and the companion (Kojima-1Lb) is a Neptune-mass
planet (20.0 \pm 2.0 M_earth) with a semi-major axis of 1.08 ^{+0.62}_{-0.18}
au. This orbit is a few times smaller than those of typical microlensing
planets and is comparable to the snow-line location at young ages. We calculate
that the a priori detection probability of Kojima-1Lb is only \sim 35%, which
may imply that Neptunes are common around the snow line, as recently suggested
by the transit and radial velocity techniques. The host star is the brightest
among the microlensing planetary systems (Ks = 13.7), offering a great
opportunity to spectroscopically characterize this system, even with current
facilities.
