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We have newly observed the Class 0/I protostar L1527 IRS using the AtacamaLarge Millimeter/submillimeter Array (ALMA) during its Cycle 1 in 220 GHz dustcontinuum and C18O (J=2-1) line emissions with a ~2 times higher angularresolution (~0.5") and ~4 times better sensitivity than our ALMA Cycle 0observations. Continuum emission shows elongation perpendicular to theassociated outflow, with a deconvolved size of 0.53"x0.15". C18O emission showssimilar elongation, indicating that both emissions trace the disk and theflattened envelope surrounding the protostar. The velocity gradient of the C18Oemission along the elongation due to rotation of the disk/envelope system isre-analyzed, identifying Keplerian rotation proportional to r^-0.5 more clearlythan the Cycle 0 observations. The Keplerian-disk radius and the dynamicalstellar mass are kinematically estimated to be ~74 AU and ~0.45 Mo,respectively. The continuum visibility is fitted by models without any annulusaveraging, revealing that the disk is in hydrostatic equilibrium. The best-fitmodel also suggests a density jump by a factor of ~5 between the disk and theenvelope, suggesting that disks around protostars can be geometricallydistinguishable from the envelope from a viewpoint of density contrast.Importantly, the disk radius geometrically identified with the density jump isconsistent with the radius kinematically estimated. Possible origin of thedensity jump due to the mass accretion from the envelope to the disk isdiscussed. C18O observations can be reproduced by the same geometricalstructures derived from the dust observations, with possible C18O freeze-outand localized C18O desorption.

ALMA Observations of the Protostar L1527 IRS: Probing Details of the Disk and the Envelope Structures