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We have observed the Class I protostar TMC-1A with AtacamaMillimeter/submillimeter Array (ALMA) in 12CO and C18O (J=2-1), and 1.3-mm dustcontinuum emission. Continuum emission with a deconvolved size of 0.50"x0.37",perpendicular to the 12CO outflow, is detected. It most likely traces acircumstellar disk around TMC-1A, as previously reported. In contrast, a moreextended structure is detected in C18O although it is still elongated with adeconvolved size of 3.3"x2.2", indicating that C18O traces mainly a flattenedenvelope surrounding the disk and the central protostar. C18O shows a clearvelocity gradient perpendicular to the outflow at higher velocities, indicativeof rotation, while an additional velocity gradient along the outflow is foundat lower velocities. The radial profile of the rotational velocity is analyzedin detail, finding that it is given as a power-law \propto r^{-a} with an indexof ~0.5 at higher velocities. This suggests that the rotation at highervelocities can be explained as Keplerian rotation orbiting a protostar with adynamical mass of 0.68 Mo (inclination-corrected). The additional velocitygradient of C18O along the outflow is considered to be mainly infall motions inthe envelope. Position-Velocity diagrams made from models consisting of aninfalling envelope and a Keplerian disk are compared with the observations,revealing that the observed infall velocity is ~0.3 times smaller than the freefall velocity yielded by the dynamical mass of the protostar. Magnetic fieldscould be responsible for the slow infall velocity. A possible scenario ofKeplerian disk formation is discussed.

ALMA Observations of the Transition from Infall Motion to Keplerian Rotation around the Late-phase Protostar TMC-1A