Incremental in-plane bending which has been invented by authors is a new and flexible manufacturing technology for small-lot production of sheet metal with various bending radii. The sheet metal is bent incrementally by an inclined-punch beating. The prototype incremental bending machine uses the numerical control technology and bends the sheet metal flexibly. The bending experiments are carried out and some experimental results such as beating force, bending radius, strain distribution are experimentally examined. The forming properties of in-plane bending are clarified in this study. Moreover, approximate formulas for calculating the bending radius are proposed base on the plane strain. The bending radius calculated by the proposed formula agrees with the one of the experiments.