PURPOSE: To easily execute the monitoring of welding condition accompanied with the automation of a welding work of an arc welding, etc., by picking up an image of the welded part with a monitoring camera by infrared ray, outputting video signals outputted from the monitoring camera with a monitor and executing the monitoring of the welded part.

CONSTITUTION: Only the infrared ray in the light coming from the welded part 6 is transmitted through an infrared ray transmitting filter 9, and since the image of the welded part 6 is picked up with the monitoring camera 8 only by the infrared ray, in the case of observing the video of the welded part 6 picked up with the monitoring camera 8 through the monitor 12 at the time of welding, the arc 10 and the position except the arc can be identified. By this method, the monitoring of the welded part 6 at the time of welding can be executed. This reason is because the extremely intense infrared ray is not generated from the arc 10 generated at the time of welding and therefore, the extremely large difference to the infrared ray quantity between the arc part 10 and the part except the arc part, is not developed.