PURPOSE: To easily manufacture the gun arm of the C-type spot welding gun of an extruded shape by notching a part to be bent in advance, then, bending the part and joining an opend part faned out towards the outside thereof.

CONSTITUTION: When the extruded shape by extrusion of a mandrel system is bent into a C type, since tensile stress concentrates on the outside corner part of the extruded shape, a side plate (a) and a side plate (b) are notched leaving the thickness (e) of a bottom plate (c) on two places concentrating the corner part in advance. When the notched extruded shape is then bent, notching is opened faned out towards the outside by this bending, a joint plate 2 having a H-shaped cross section is inserted into the fan-shaped opening and this is welded to the side plate side and the partition plate (d) side so as to attain the same plane as a side plate plane. Both side plate upper parts (f) rising outside from the partition plate (d) are cut or machined into an arm shape and the C type gun arm can be easily manufactured.