PURPOSE: To control a die gap basing on the set value and to mass produce a tapered rod, by drawing a material to be worked from a gap of roller dies opposing to each other, and detection sectional dimensions at a prescribed point.

CONSTITUTION: A wire rod diam. detector 3 is provided to the point being a prescribed distnace away, in the advancing direction of a material to be worked, from roller dies 1 and 1'. As the dies 1 and 1' and the detector 3 are separated by the prescribed distance, a taper setter 5 transmits the die-gap-computed data to an arithmatic device 6 so as to increase a die gap in a range of A of the material M, to decrease the gap in ranges of B and C, to increase the gap in a range of D. Next, the die gap value is computed with the device 6 by substituting a diametral value of the material M, inputted to the device 6 through A and D converters 4, into said data, and a controller 7 outputs a control-driving signal to a screw down mechanism 10 through a hydraulic pulse motor 9 so as to bring the die gap close to the computed value.