An electrically heating device is provided with a pair of clamp units serving as electrodes for energization and applying tension to a workpiece to be heated clamped between the clamp units by moving the clamp units in the direction in which the clamp units separate from each other during energization heating. The clamp units each include a facing surface inclined with respect to the moving direction of the clamp units and form, by the facing surface, a preform including an inclined surface inclined with respect to the plane direction of the workpiece in a clamping portion clamped between the clamp units in the workpiece.