  We have observed the Class I protostar TMC-1A with Atacama
Millimeter/submillimeter Array (ALMA) in 12CO and C18O (J=2-1), and 1.3-mm dust
continuum emission. Continuum emission with a deconvolved size of 0.50"x0.37",
perpendicular to the 12CO outflow, is detected. It most likely traces a
circumstellar disk around TMC-1A, as previously reported. In contrast, a more
extended structure is detected in C18O although it is still elongated with a
deconvolved size of 3.3"x2.2", indicating that C18O traces mainly a flattened
envelope surrounding the disk and the central protostar. C18O shows a clear
velocity gradient perpendicular to the outflow at higher velocities, indicative
of rotation, while an additional velocity gradient along the outflow is found
at lower velocities. The radial profile of the rotational velocity is analyzed
in detail, finding that it is given as a power-law \propto r^{-a} with an index
of ~0.5 at higher velocities. This suggests that the rotation at higher
velocities can be explained as Keplerian rotation orbiting a protostar with a
dynamical mass of 0.68 Mo (inclination-corrected). The additional velocity
gradient of C18O along the outflow is considered to be mainly infall motions in
the envelope. Position-Velocity diagrams made from models consisting of an
infalling envelope and a Keplerian disk are compared with the observations,
revealing that the observed infall velocity is ~0.3 times smaller than the free
fall velocity yielded by the dynamical mass of the protostar. Magnetic fields
could be responsible for the slow infall velocity. A possible scenario of
Keplerian disk formation is discussed.
