PURPOSE: To accurately sense the position of the edge of a web even if it is easy to break or is transparent

CONSTITUTION: A radiation thermometer 16 is arranged on the upper side of a transport path for a web 12. The radiation thermometer 16 is supported slidably by a rail 18 laid perpendicularly to the direction across the width of the web 12, installed over the transport path 14 for the web 12, and slided along the rail 18 by a drive motor. The rail 18 is extended to an outside position from the transport path 14, and therewith the thermometer 16 senses the surface temp. of the web 12 and the heater 20 surface temp. which has been set some higher than that of the web 12. The two temps. obtained are fed to a CPU 22. The CPU 22 senses the edge positions of the web 12 on the basis of these temps. and corrects meander of the web 12 in case the measured edge position deviates from the tolerance according to the reference edge position.