Method and device for manufacturing vehicle arm component

To provide a vehicle arm component manufacturing method whereby the yield of a material can be further improved. The present invention has: an expanding step (S05) for expanding the X-direction widthof a workpiece from a first width (X1) to a second width (X2) by pressing, in the Z direction, a portion (P8) to be applied with pressure, said portion being provided within the surface of the workpiece (W); and a separating step (S06) for cutting down, from a material (M) to be machined, the workpiece the X-direction width of which has been expanded to the first width. 提供种能够进步提高材料的利用率的车辆用臂部件的制造方法。包括：扩展工序(S05)，通过在Z方向上对设置于工件(W)的面内的被加压部(P8)进行冲压，使工件在X方向上的宽度从第宽度(X1)扩展至第二宽度(X2)；分离工序(S06)，将在X方向上的宽度被扩展至第宽度的工件从被加工材料(M)切下。