During the process of bending a workpiece, actual load exerted on the workpiece and displacement are sampled and the bending stress to displacement relationship is obtained from the sampling data and data on the actual thickness of the workpiece. From the bending stress to displacement relationship, a material characteristic value (e.g., yield stress) which indicates the condition of the material of the workpiece is obtained. Then, a bending angle correction amount is obtained from the material characteristic value, using a data table concerning the material characteristic value to bending angle relationship. From the bending angle correction amount thus obtained, a correction depth amount is obtained. The correction depth amount is added to a preset reference depth amount to obtain a final depth amount according to which bending is performed.