  We re-process the Atacama Large Millimeter/Submillimeter Array (ALMA)
long-baseline science verification data taken toward HL Tauri. As shown by the
previous work, we confirm that the high spatial resolution (~ 0."019,
corresponding to ~ 2.7 AU) dust continuum images at \lambda = 0.87, 1.3, and
2.9 mm exhibit a multiple ring-like gap structure in the circumstellar disk.
Assuming that the observed gaps are opened up by currently forming, unseen
bodies, we estimate the mass of such hypothetical bodies based on following two
approaches; the Hill radius analysis and a more elaborated approach developed
from the angular momentum transfer analysis in gas disks. For the former, the
measured gap widths are used for calibrating the mass of the bodies, while for
the latter, the measured gap depths are utilized. We show that their masses are
likely comparable to or less than the mass of Jovian planets, and then discuss
an origin of the observed gap structure. By evaluating Toomre's gravitational
instability (GI) condition and cooling effect, we find that the GI might be a
possible mechanism to form the bodies in the outer region of the disk. As the
disk might be gravitationally unstable only in the outer region of the disk,
inward planetary migration would be needed to construct the current
architecture of the hypothetical bodies. We estimate the gap-opening mass and
show that type II migration might be able to play such a role. Combining GIs
with inward migration, we conjecture that all of the observed gaps may be a
consequence of bodies that might have originally formed at the outer part of
the disk, and have subsequently migrated to the current locations. While ALMA's
unprecedented high spatial resolution observations can revolutionize our
picture of planet formation, more dedicated observational and theoretical
studies are needed in order to fully understand the HL Tau images.
