To prevent leakage of cooling water from between a holder and an adapter by providing an O-ring fitted into an O-ring groove and a taper fit in such a manner that both exhibit a sealing effect.

The structure of the electrode attaching and detaching section of a welding gun 10 is formed by attaching a holder 12B to a gun arm 11 and attachably and detachably attaching the adapter 14 to the holder 12B. The holder 12B is provided with an internal thread section 32, a tapered section 33B which is disposed deeper than the internal thread section, has the diameter smaller than the diameter of the internal thread section and has the diameter diminishing as this section parts increasingly from the internal thread section and the O-ring groove 34 disposed in mid-way of an internal thread tapered section. The adapter 14 is provided with an external thread section 37 to be screwed into the internal thread section and the external tapered section 38 to be taper-inserted into the internal thread section.