To provide a heater apparatus 23 for use in performing a bending operation through heating and cooling of a steel member 1 of grooved sectional surface capable of performing an efficient heating operation and hardly producing any twisted state.

A web 3 having a grooved shape and a profiling guide 27 of substantial <-shape along a surface shape of a flange 5 are mounted on steel member having a groove-shape. A moving segment 31 is moved. The moving segment 31 has a plurality of rollers 55, 57 at each of right and left portions, respectively. The rollers are press contacted with an upper surface 41 of the profiling guide and an upper edge 43 of a guide longitudinal hole 39 and rotated. The moving segment is connected to a part of a chain 65 driven along a plurality of sprockets 59, 61 and 63 arranged at an outer edge of the guide longitudinal hole. The moving segment is provided with a driving means for performing a reciprocating motion. A plurality of heating torches 25 are fixed to the reciprocating segment.