The invention has as its object to perform hydroforming so that no buckling or wrinkles remain at a hydroformed product with a long expanded region and comprises performing a first step of raising the internal pressure in a state with the metal tube fixed in position at the two ends or a state applying axial pushing actions of 10% or less of the total amount of axial pushing action, then applying axial pushing actions while holding the internal pressure at a constant pressure so as to expand the metal tube near the ends, then performing a second step of raising only the internal pressure without applying any axial pushing action so as to thereby expand a center of the metal tube, then performing a third step of lowering only the internal pressure to the value of the constant pressure without applying any axial pushing action, then repeating the first to third steps one or more times, then raising the internal pressure in the state not applying any axial pushing action or applying an axial pushing action of 10% of the total axial pushing action amount or less.