A tube expanding method for manufacturing a metal tube such that, to an internal processing tool having a taper portion, a shoulder portion, and a parallel portion that are smoothly connected each other in this order from a starting end that is the minimum diameter end of the tool to a terminal end that is the maximum diameter end of the tool, a metal tube that is a workpiece whose hollow piece internal diameter is smaller than the diameter of the parallel portion, that is, the maximum diameter of the tool is fed, the starting end side of the tool being used as an entrance side, the internal diameter side of the tube is caused to pass through the tool, and thereby the workpiece undergoes tube expanding processing, the tube expanding method being characterized in that the size of the hollow piece that is the workpiece before tube expanding processing, and size of the tool are determined by the following procedure through a tube expanding processing simulation based on the finite element analysis: (a) a target external diameter and a target thickness of a product, a length of the taper portion of the tool, and an upper load that is the upper limit of feed power of a tube expanding processing apparatus to be used are set; (b) as size parameters, an external diameter of the hollow piece, a thickness of the hollow piece, a taper angle that is a taper angle of the taper portion of the tool, a shoulder portion R that is a radius of curvature of the shoulder portion in the axial section of the tool, and the maximum diameter of the tool that is the diameter of the parallel portion of the tool are set; (c) the workpiece is divided into a plurality of elements; (d) tube expanding processing simulation is performed, and a calculated load that is a calculated feed power, and an external diameter after processing, a thickness after processing, and an internal diameter after processing of the workpiece when a tip of the workpiece has advanced by 1.0 m or more from the shoulder portion position toward a tool exit side are obtained; (e) it is determined whether or not the calculated load is less than or equal to the upper load; if the determination result is Yes, step (f) is proceeded to; if it is No, the thickness of the hollow piece, or the thickness of the hollow piece and the taper angle are changed, and step (c) is returned to; (f) it is determined whether or not the external diameter after processing is within a range of the target external diameter ±1% and the thickness after processing is within a range of the target thickness ±7.5%; if the determination result is Yes, step (g) is proceeded to; if it is No, the external diameter of the hollow piece and/or the thickness of the hollow piece is changed, and step (c) is returned to; and (g) it is determined whether or not an internal gap = the internal diameter after processing - the maximum diameter of the tool is 1 mm or less; if the determination result is Yes, the size parameters at this time are determined to be size parameters for actual tube expanding manufacturing, and the procedure is ended; if it is No, the shoulder portion R and/or the taper angle is changed, and step (c), (d) is returned to. (a) a target external diameter and a target thickness of a product, a length of the taper portion of the tool, and an upper load that is the upper limit of feed power of a tube expanding processing apparatus to be used are set; (b) as size parameters, an external diameter of the hollow piece, a thickness of the hollow piece, a taper angle that is a taper angle of the taper portion of the tool, a shoulder portion R that is a radius of curvature of the shoulder portion in the axial section of the tool, and the maximum diameter of the tool that is the diameter of the parallel portion of the tool are set; (c) the workpiece is divided into a plurality of elements; (d) tube expanding processing simulation is performed, and a calculated load that is a calculated feed power, and an external diameter after processing, a thickness after processing, and an internal diameter after processing of the workpiece when a tip of the workpiece has advanced by 1.0 m or more from the shoulder portion position toward a tool exit side are obtained; (e) it is determined whether or not the calculated load is less than or equal to the upper load; if the determination result is Yes, step (f) is proceeded to; if it is No, the thickness of the hollow piece, or the thickness of the hollow piece and the taper angle are changed, and step (c) is returned to; (f) it is determined whether or not the external diameter after processing is within a range of the target external diameter ±1% and the thickness after processing is within a range of the target thickness ±7.5%; if the determination result is Yes, step (g) is proceeded to; if it is No, the external diameter of the hollow piece and/or the thickness of the hollow piece is changed, and step (c) is returned to; and (g) it is determined whether or not an internal gap = the internal diameter after processing - the maximum diameter of the tool is 1 mm or less; if the determination result is Yes, the size parameters at this time are determined to be size parameters for actual tube expanding manufacturing, and the procedure is ended; if it is No, the shoulder portion R and/or the taper angle is changed, and step (c), (d) is returned to.