  Deuterium fractionation in the closest vicinity of a protostar is important
in understanding its potential heritage to a planetary system. Here, we have
detected the spectral line emission of CH3OH and its three deuterated species,
CH2DOH, CHD2OH, and CH3OD, toward the low-mass protostellar source B335 at a
resolution of 0.''03 (5 au) with Atacama Large Millimeter/submillimeter Array.
They have a ring distribution within the radius of 24 au with the intensity
depression at the continuum peak. We derive the column densities and abundance
ratios of the above species at 6 positions in the disk/envelope system as well
as the continuum peak. The D/H ratio of CH3OH is ~[0.03-0.13], which is derived
by correcting the statistical weight of 3 for CH2DOH. The [CHD2OH]/[CH2DOH]
ratio is derived to be higher ([0.14-0.29]). On the other hand, the
[CH2DOH]/[CH3OD] ratio ([4.9-15]) is higher than the statistical ratio of 3,
and is comparable to those reported for other low-mass sources. We study the
physical structure on a few au scale in B335 by analyzing the CH3OH
(183,15-182,16, A) and HCOOH (120,12-110,11) line emission. Velocity structures
of these lines are reasonably explained as the infalling-rotating motion. The
protostellar mass and the upper limit to centrifugal barrier are thus derived
to be 0.03-0.07 M_{\odot} and <7 au, respectively, showing that B335 harbors a
young protostar with a tiny disk structure. Such youth of the protostar may be
related to the relatively high [CH2DOH]/[CH3OH] ratio.
