PURPOSE: To obtain a titled device which can be brought easily to maintenance and can detect exactly an abnormal state, by constituting so that a current of arc welding is detected through a transformer, a light emitting diode is lighted by its output, and it is sent as an optical signal to a discriminating circuit.

CONSTITUTION: A cap pin 4 of a fluorescent lamp 2 which is installed to a conveyor 1 and is carried is made to press-contact onto a conductor 6 by a pin holder 15, welding voltage is impressed from a welding power source 8 having a controlling circuit, through a cable 9, between a welding electrode 7 opposed with a space distance of about 0.5W2mm to the tip of the cap pin 4, and the cap pin 4 and a lead wire 5 which is inserted through said pin and is projected from the tip are arc-welded. By a current of this arc discharge, a light emitting diode 11 is emitted through a current transformer 10 added to the cable 9, and its optical signal is inputted to a signal processing circuit 13 together with signals from a limit switch 14, the controlling circuit 8, etc., through an optical transmission cable 12 consisting of a fiber, by which whether abnormality of a welding current exists or not is discriminated.