An asymmetric parallel return flow, which modifies the parallel component of the flow, is expected to meet the zero divergence of the flow on a flux surface based on the common neoclassical theory for torus plasma. Full flow structure is measured by charge exchange spectroscopy on LHD. Inboard/outboard asymmetry of parallel flow is observed according to the full flow profile measurement. Flow asymmetry is considered to be induced by the Pfirsch-Schlüter flow closely associated with the radial electric field. A linear relationship between the integrated flow asymmetry and the electric potential difference is obtained in different magnetic fields and configurations. A model based upon the incompressibility of the flow is applied to acquire a geometric factor hB, which only connects to magnetic configuration from experiment. Asymmetric component of parallel flow measured is compared with the asymmetric component of parallel flow calculated in the incompressibility condition of flow on the magnetic flux surface. The measured asymmetric flow is consistent with the calculation in the plasma with small toroidal torque input in the inward shifted configuration. However, the measured asymmetric flow is significantly smaller than that calculated for the plasma with large toroidal torque or in the outward shifted configuration. One possible candidate could be the radial transport due to anomalous perpendicular viscosity plus strongly poloidally asymmetric radial flow.