PURPOSE: To grasp accurately a condition of the wall thickness deviation of a pipe and the causes of its generation by continuously measuring a vibrating condition in the longitudinal direction of a blank pipe in rolling and monitoring the condition of wall thickness deviation basing of the periodicity of the vibration.
CONSTITUTION: A line sensing camera 6 for monitoring a pipe 2 in rolling and a hot steel detector 4 for detecting the front and rear ends of the pipe 2 are provided to the outlet sides of an elongator 1 for elongating and reducing its wall thickness by a mandrel 3. Next, when a pipe size and rolling conditions, etc. are previously inputted, as an external information 10, to a processing device 8, and a dislacement of pipe 2 is took in the device 8 by the camera 6 basing on its radiant light; a Fourier transducer 7 samples by a prescribed pitch the analogized signals obtained from the camera 6 through a controller 5 to obtain the periodic spectrum of them by Fourier transformation, and the device 8 performs the frequency analysis to output the results to a printer 9 or an alarm. In this way, a condition of the wall thickness deviation of a blank pipe during rolling is accurately grasped and the causes of the deviation are searched.