To provide a hydraulic forming apparatus capable of easily forming a flanged tubular member, and controlling the thickness of the tubular member.

This hydraulic forming apparatus comprises a male die 20 which is relatively moved to a tubular stock 10 having a closed section and a plate overlapping part 14 outside the closed section, a female dies 30 which is split into a plurality of parts, and holds and fixes the overlapping part of the tubular stock, a holding tool 40 for hydraulically sealing an outer periphery at both ends of the tubular stock, and a shaft cylinder tool 45 having a nozzle 46 for feeding and discharging working fluid, inserted in both ends of the tubular stock, and holding and sealing the tubular stock between the holding tool and itself, and forms the flanged tubular member having the peripheral length of the closed section shorter than that of the tubular stock with the thickness of at least one side of the closed section being larger than the thickness of the tubular stock.