PURPOSE: To reduce the size of a device and to improve its reliability by installing an infrared sensor means except at a prove follow-up mechanism and connecting a condenser lens provided to the probe follow-up mechanism to the infrared sensor by an optical fiber.

CONSTITUTION: Two infrared-ray radiation type temperature sensors 30 are used and respective sensor heads 31 are connected by optical fibers 32. Infrared rays of extremely small areas P1 and P2 of a seam welded pipe 4 are converged by lenses of the heads 31 and transmitted to sensors 30 through the optical fibers 32. The infrared rays are intermitted by a mechanical chopper 33 to enter an infrared -ray detecting element 34 and specific signal processing is carried out by an electronic circuit 35 on the basis of the element 34, so that the quantity of displacement of a weld zone 3 is outputted to a servo-controller. Thus, the device is reduced in size and the reliability is improved.