For aging deformation of a reaction vessel used for production of titanium sponge by the Kroll method, the deformation of the reaction vessel can be corrected to a desired deformation. The apparatus for correcting the deformation corrects by being inserted inside of the cylindrical deformation of the reaction vessel, the apparatus has multiple cylinder arms radially extendable to a circumference, a deformation-correcting head arranged on a top part of the cylinder arm, a hydraulic power unit connected to the cylinder arm and driving the deformation-correcting head, a detecting means for the stroke of the deformation-correcting head, and a measuring means for the pressing force against the reaction vessel. Furthermore, the method for correcting the deformation of the reaction vessel using the apparatus has a step of pressing the reaction vessel while adjusting stroke of the deformation-correcting head depending on an amount of deformation of the reaction vessel.