PURPOSE: To provide a device which is capable of forming a curved surface that accurately matches a metallic die at the time of the three-dimensional bending of a metallic sheet.

CONSTITUTION: On both sides in the longitudinal direction of a metallic die with a curved surface in the width direction, a first clamping device 14 and a second clamping device 15 each for clamping both ends of a metallic sheet are arranged ; the device is so structured that at least either one of the clamping devices 14 or 15 is moved in the longitudinal direction; the first clamping device 14 or the second clamping device 15 is formed as two or more unit clamping mechanism groups 17a-17f divided in the width direction; and the stretch former is provided with a tension adjusting mechanism in each unit clamping mechanism 17.