1. A method for determining a heating point in the bending of a steel plate, comprising: determining the number of a plurality of congruent isosceles triangles, which are connected together while sharing their equal sides, on the basis of the radius of a curve of a target shape of the steel plate to be bent, the radius of a curve of a measured shape of the steel plate, and a separately set bending angle of the steel plate so that when the curve of the target shape of the steel plate is regarded as an arc, the arc of the target shape of the steel plate can be approximated by a fold line defined by the bases of the plural congruent isosceles triangles and that when the curve of the measured shape of the steel plate is regarded as an arc, the arc of the measured shape of the steel plate can be approximated by a fold line defined by the bases of a plurality of other congruent isosceles triangles which are connected together while sharing their equal sides, the number of the latter isosceles triangles being the same as the number of the former isosceles triangles whose bases constitute the approximating fold line for the target shape; dividing the arc of the measured shape by the number of the isosceles triangles to form respective points on the arc; and using the respective points as heating points.