We report the detection of a non-corotating gas component in a bright unlensed submillimeter galaxy at z = 4.3 , COSMOS-AzTEC-1 , hosting a compact starburst . ALMA 0.17 and 0.09 arcsec resolution observations of [ C ii ] emission clearly demonstrate that the gas kinematics is characterized by an ordered rotation . After subtracting the best-fit model of a rotating disk , we kinematically identify two residual components in the channel maps . Both observing simulations and analysis of dirty images confirm that these two subcomponents are not artificially created by noise fluctuations and beam deconvolution . One of the two has a velocity offset of 200 km s ^ { -1 } and a physical separation of 2 kpc from the primary disk and is located along the kinematic minor axis of disk rotation . We conclude that this gas component is falling into the galaxy from a direction perpendicular to the disk rotation . The accretion of such small non-corotating gas components could stimulate violent disk instability , driving radial gas inflows into the center of galaxies and leading to formation of in-situ clumps such as identified in dust continuum and CO. We require more theoretical studies on high gas fraction mergers with mass ratio of 1 : > 10 to verify this process .