The thermal transpiration effect, which means a significant pressure difference between the two ends of a pipe due to a large temperature difference, has been discussed in ion mobility measurements in helium gas at 4.3 K. A modified Takaishi-Sensui's empirical formula for the pressure correction is obtained from the experimental results. We propose to use this formula instead of the original Takaishi-Sensui equation for helium gas. By using the new formula, the reduced mobilities of He-4(+) and Ar-40(+) ions in He-4 gas have been obtained with the measurements of drift velocities at the gas temperature of 4.3 K and comparisons have been made with the recent theoretical calculations by Dickinson [J. Phys. B: At. Mol. Opt. Phys. 32, 4919 (1999)] and Viehland , [J. Chem. Phys. 105, 11143 (1996)], respectively, which show fairly good agreements with the experimental results.