This paper describes a production process for experiment and finite element method (FEM) analysis of cold forming of SUS304 pipe. These large diameter pipes such as f114.3 mm are used for a plant as a flow channel of gas and liquid. The connection of pipes are generally welded at the plant. However, the other connecting method are required from a viewpoint of making the plant environment worse by welding. Therefore, flaring process of large diameter pipes were proposed. This flaring process is one of a method of pipe flange forming. The formed pipes were connected used with loose flange. Flaring process was generally hot process, thus it has some problem such as becoming complex of forming machine and accuracy of dimension. In this study, cold flaring process of SUS304 pipe was proposed to satisfy these requisitions. Experiment and FEM analysis of cold flaring process were performed to clarify the optimum forming conditions for the flat length of connecting surface such as a diameter of punch, punch stroke and taper angle of dies. As a result, a gap between punch and die was needed to match the pipe wall thickness.