Abstract: There can be provided a spot welding gun in which a gun arm can be detachably attached to a gun main body, capable of conducting welding operation without increase in size and with good precision. Coupling portions 7 used to attach/detach the gun arm 9 having the fixed electrode 13 to/from the gun main body 1 having the pressure electrode 11 comprise an insertion recess portion which is formed on the gun main body 1 side to be tapered along the direction perpendicular to pressure direction of the pressure electrode 11, and an insertion portion which is formed on the gun arm 9 side to be tapered along the direction perpendicular to the pressure direction for easy insertion into the insertion recess portion.