Each of a pair of opposing slides which is equipped with a die and is connected through a connecting rod to an eccentric portion of a crankshaft is displaced by a width setting device in the direction of width of a slab so as to set the width of a slab to be forged between the pair of dies. After the setting of the width of a slab to be forged, the crankshaft is rotated to cause each of the slider-pair to reciprocate not only in the direction of width of the slab but also in the direction in parallel with the slab transportation line, whereby a slab is forged.