The invention provides a system and method for monitoring the molten state of electric-resistance-welded pipe on-line. In the system, a mirror provided at a front end side inside a container receives light emitted by the welded parts of the tube-shaped steel strip through heat resistant glass from the side direction and reflects the image in the direction of a relay lens provided at a base end side of a relay lens unit. The relay lenses relay the image to a conversion lens, which forms the image on the imaging area of a CCD camera. Therefore, it is possible to capture the welded parts from the side direction by a resolution in accordance with the resolution of the CCD camera and possible to obtain information of the welded parts more accurately than in the past and on-line.