PURPOSE: A nut welding device is provided to stably execute welding work of a nut by detecting an optical signal from a luminescent lamp according to the position of an interlock shaft and determining a loading state of the nut. CONSTITUTION: A nut welding device includes a lower holder(3) formed to the lower part of a body(1) and an upper holder(7) mounted to the front end of a rod(11) of an operating cylinder(9) mounted to the upper part of the body correspondently to the lower holder, to press a weld nut(13) to be welded to a panel material(10) to be loaded on the lower holder and apply an electric current. The nut welding device comprises a shaft guide block(19) wherein a light transmission hole(21) is formed in the horizontal direction and a guide hole(23) overlapping with the light transmission hole is formed in the longitudinal direction; an interlock shaft(25) slidably fitted to the guide hole of the shaft guide block; a sensing unit(29) installed to an inner side of the shaft guide block correspondently to the light transmission hole, to receive an optical signal and to output a detection signal to a control part; and a luminescent unit(31) mounted to a side of the operating cylinder through a mounting bracket correspondently to the other side of the light transmission hole of the shaft guide block. The shaft guide block is mounted to one side of the upper part of the body. One end of the interlock shaft is fixed to one side of the rod of the operating cylinder and a connection hole, bored parallel with the light transmission hole of the shaft guide block, is formed to the other side of the interlock shaft.
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