The luminescence from excited helium atoms is measured on a small tokamak Hamana-T. The exponential growth rate of the luminescence is maximized when the resultant effect of the toroidal and the error field drifts of electrons is compensated by the optimum value Bvo of external vertical magnetic field. The error vertical magnetic field Bve and the equivalent vertical magnetic field Bvd, which yields the same displacement as the toroidal drift and is proportional to toroidal magnetic field Bt, are experimentally determined by the values of Bvo obtained with different values of Bt and electric field E. The field Bvd is approximately proportional to E1/2.