A 1.24 MeV deuteron (D) beam mixed with a H 2 molecular beam was separated with a microslit system of a nuclear microprobe consisting of a 100 μm diameter object and a 1 mm diameter aperture diaphragm. D was distinguished from H 2 by Rutherford backscattering (RBS) on a thin Au film. By slightly changing the magnetic field strength of the beam steerer installed in front of the object diaphragm, the maximum and the minimum RBS D/H 2 ratios were found to be 50.3 and 1.5, respectively. M/ ΔM = 3.9 × 10 3 was obtained as the mass resolution of the nuclear microprobe. The transmission of this system was 2 × 10 −3.