[en] The sizes of sample volumes, which determine the spatial resolution, are estimated by trajectory calculation for the heavy ion beam probe on LHD. The beam width, the divergence, and the beam energy difference are taken into account. From the points of view of the spatial resolution and the signal intensity, parallel beam is suitable for the injection beam. The size of the sample volume by the parallel beam with 10 mm diameter is 42 mm. It is sufficient to measure the potential profile in the internal transport barrier in the LHD plasma. Taking into account the spatial profile of the beam, the spatial resolution will be better than 42 mm. (author)