Abstract: An apparatus according to the present invention comprises a fixed electrode rod and a movable electrode unit movable toward and away from the fixed electrode rod. The movable electrode unit includes a first electrode rod for holding a set of workpieces with a predetermined force in cooperation with the fixed electrode rod, and a second electrode rod in contact with the first electrode rod. Each electrode rod is covered by means of a water jacket, and cooling water is continually fed into the respective water jackets of the rods during the operation of the apparatus. When energized, the first and second electrode rods generate heat on the basis of contact resistance between them, resisting the cooling effect of the water jackets. As a result, the workpieces held between the first electrode rod and the fixed electrode rod are heated to a predetermined temperature.