We studied low-energy (similar to 1.55keV) electron-spin-polarized He-4(+) ion scattering on a Bi(111) ultrathin film epitaxially grown on a Si(111) substrate. We observed that the scattered ion intensity differed between the incident He+ ions with up and down spins even though Bi is a non-magnetic element. To analyze the origin of this spin-dependent ion scattering (the spin asymmetry), we investigated the detailed relationship between the spin asymmetry and the incident angle, the azimuthal angle, the scattering angle, and the incident energy. All the data indicate that the spin asymmetry originates from the scattering cross section owing to the non-central force in the He+-Bi atom binary collision. The non-central force is most likely attributed to the spin-orbit coupling that acts transiently on the He+ 1s electron spin in the binary collision.