A high-performance liquid chromatograph apparatus of the present invention can obtain a similar separation effect as that obtained in a constant velocity gradient elution method and shorten a measuring time. When a data processor (13) generates a converted constant pressure gradient program based on a trace of past pressure values, a pressure value trace file stored in a pressure value trace storage unit (13a) is selected and displayed on a display and a pressure value is set. A constant pressure gradient program converting unit (13b) converts a constant velocity gradient program stored in a constant velocity gradient program storage unit (13c) to the constant pressure gradient program and numerals according to the converted constant pressure gradient program are displayed. A liquid feed part (5, 6) of a liquid feed pump (4) is controlled via a flow velocity setting unit (13d) and a mixing ratio setting unit (13e) according to the constant pressure gradient program displayed on the display.