For measuring the source profile of proton beams, a method using a Thomson mass spectrometer, which has precise energy resolution and enables the discrimination of ion species, was proposed and applied in an experiment with irradiation of 55-fs, 6.6× 1018 -W cm2 -laser pulses on a 5-μm -thick copper tape target. The typical measured source size was 150 μm at E=400 keV and 44 μm at E=980 keV in full width at half maximum.