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Revealing gas kinematics around protostars is crucial to understand the process of star and disk formation. Observational studies have characterized gas motions around protostars; protostars are generally surrounded by infalling envelopes (~1000 au) and rotationally supported disks (~100 au). However, more comprehensive picture of the kinematic structure over a wide spatial range from a core scale (~0.05 pc) to a disk scale is not yet obtained. In this talk, I will present observational studies on protostars, in which I investigated the gas kinematics over the wide spatial scale. Velocity structures on different spatial scales are characterized by measuring the radial dependence of the peak velocity of line emission. The peak velocity approximately follows Keplerian rotation and rotation with a constant specific angular momentum, which is suggestive of the infalling envelope, at radii less than a few thousands au, as was also reported in some other protostellar systems. On the other hand, the peak velocity is proportional to r^~0.6 at larger radii, which resembles the velocity structures of the initial dense cores (J/M~r^1.6). These results give rough size scales of the infalling envelopes of ~1400 to 2900 au around individual protostars, and suggest that the spatial scale possibly increases as the system evolves. I introduced the structure function analysis to probe velocity structures at larger radii, implying that the velocity structures at larger scales originate from turbulent motion. Video call link: https://meet.google.com/ydj-unof-qbg