We present combined Submillimeter Array (SMA)+Atacama Submillimeter Telescope Experiment (ASTE) images of the Class I protobinary L1551 IRS 5 in the CS (J = 7-6) line, the submillimeter images of L1551 IRS 5 with the most complete spatial sampling ever achieved (0".9-36"). The SMA image of L1551 IRS 5 in the 343 GHz dust-continuum emission is also presented, which shows an elongated feature along the northwest to southeast direction (~160 AU x 80 AU), perpendicular to the associated radio jets. The combined SMA+ASTE images show that the high-velocity (gap1.5 km s super(-1)) CS emission traces the structure of the dust component and shows a velocity gradient along the major axis, which is reproduced by a geometrically thin Keplerian-disk model with a central stellar mass of ~0.5 M sub(middot in circle). The low-velocity (lap1.3 km s super(-1)) CS emission shows an extended (~1000 AU) feature that exhibits slight south (blueshifted) to north (redshifted) emission offsets, which is modeled with a rotating and infalling envelope with a conserved angular momentum. The rotational motion of the envelope connects smoothly to the inner Keplerian rotation at a radius of ~64 AU. The infalling velocity of the envelope is ~three times lower than the free-fall velocity toward the central stellar mass of 0.5 M sub(middot in circle). These results demonstrate transition from the infalling envelope to the Keplerian disk, consistent with the latest theoretical studies of disk formation. We suggest that sizable (r ~ 50-200 AU) Keplerian disks are already formed when the protostars are still deeply embedded in the envelopes.