Abstract: For the separation of a peptide or protein in a sample using a supercritical fluid chromatograph, carbon dioxide is used as the supercritical fluid, while an organic solvent containing hexafluoroisopropanol is used as the modifier to be added to the supercritical fluid. If a mixed liquid of methanol, isopropanol, water and formic acid is simply used as the modifier, a considerable amount of peak tailing occurs on the chromatogram, causing the peak waveform to be distorted. By comparison, when a mixed liquid prepared by additionally mixing hexafluoroisopropanol in the mixed liquid of methanol, isopropanol, water and formic acid is used, the peak observed on the chromatogram will have a sharp, satisfactory shape.