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We report Atacama Large Millimeter/submillimeter Array (ALMA) cycle 0observations of C$^{18}$O ($J=2-1$), SO ($J_N= 6_5-5_4$) and 1.3mm dustcontinuum toward L1527 IRS, a class 0 solar-type protostar surrounded by aninfalling and rotating envelope. C$^{18}$O emission shows strong redshiftedabsorption against the bright continuum emission associated with L1527 IRS,strongly suggesting infall motions in the C$^{18}$O envelope. The C$^{18}$Oenvelope also rotates with a velocity mostly proportional to $r^{-1}$, where$r$ is the radius, while the rotation profile at the innermost radius (54 AU)may be shallower than $r^{-1}$, suggestive of formation of a Keplerian diskaround the central protostar of 0.3 Mo in dynamical mass. SO emission arisingfrom the inner part of the C$^{18}$O envelope also shows rotation in the samedirection as the C$^{18}$O envelope. The rotation is, however, rigid-body likewhich is very different from the differential rotation shown by C$^{18}$O. Inorder to explain the line profiles and the position-velocity (PV) diagrams ofC$^{18}$O and SO observed, simple models composed of an infalling envelopesurrounding a Keplerian disk of 54 AU in radius orbiting a star of 0.3 Mo areexamined. It is found that in order to reproduce characteristic features of theobserved line profiles and PV diagrams, the infall velocity in the model has tobe smaller than the free-fall velocity yielded by a star of 0.3 Mo. Possiblereasons for the reduced infall velocities are discussed.

Formation of a Keplerian disk in the infalling envelope around L1527 IRS: transformation from infalling motions to Kepler motions