To facilitate the control of a forming gas pressure, to shorten forming time, to improve forming precission and to simplify an apparatus configuration by uniforming the forming rate of a super-plastic diffusion-bonding panel.

Plates 51, 52 for forming the face surface of a panel are housed in a forming die 30 and superplastic forming is performed by moving the plates 51, 52 in accordance with the expansion of the panel while pressing the plates 51, 52 against face sheets 2, 3 by using means such as gas pressure, a spring and a metallic honeycomb.