Abstract: Arranged in tandem are a pre-heating unit 6, a four-side hot-rolling mill 7 and a quenching/cooling unit 8. A starting material W1 is rapidly heated to a hot-rolling temperature using the unit 6 and hot-rolled into a modified cross-section steel wire W2 assuming a predetermined shape in cross section using the mill 7. The wire is then quenched under tension using the unit 8 immediately after its hot-rolling operation. The tension is applied to the wire by a pinch roll 9 driven by a motor 9a, which is combined with a torque converter. Consequently, there is no variation in tension, and no variation in cross-sectional dimension in the wire. Disposed behind the unit 8 in tandem is an in-line post-heating coil 10 to realize a continuous tempering operation, so that the wire free from any small-radius bend and having been heat-treated, can be produced through a substantially single process.