To form bilaterally symmetrical dog-bone shapes of the edge parts of a web stock without decreasing the operation rate of a production line.

An apparatus 11 is directed to form the edge parts into bilaterally symmetrical dog-bone shapes before welding a web stock 1w and a flange stock 1f, by conducting rolling-reduction of the edge parts of the web stock 1w using a pair of pre-setup rolls 12a, 12b, in which calibers 12aa, 12ba are formed, when manufacturing a welded H-section steel. A plurality of calibers 12aa, 12ba are formed according to the thickness of the web stock 1w at the pre-setup rolls 12a, 12b. An axial movement mechanism 13 for moving the pre-setup rolls 12a, 12b is provided. The pre-setup rolls 12a, 12b are moved according to the thickness of the web stock 1w in the axial direction by driving the axial movement mechanism 13, the calibers 12aa, 12ba that are optimum for the thickness of the web stock 1w are selected, and formation of the edge parts of the web stock 1w are conducted. Thereby, the bilaterally symmetrical dog-bone shapes are formed on the edge parts of the web stock without decreasing the operation rate of a production line.