PURPOSE: To measure tension without stopping an apparatus and to enhance operating efficiency, by injecting a fluid to the inner peripheral edge of an inner peripheral blade or sucking air from the vicinity of said inner peripheral edge to bent the inner peripheral blade in a state not brought into contact with a measuring device and reading the displacement quantity thereof by a displacement meter.

CONSTITUTION: An electropneumatic proportional valve 8 is opened by the order from a control apparatus and air of predetermined pressure is injected from a jet nozzle 1-1 to press the vicinity of the inner peripheral edge 3 of an inner peripheral blade downwardly. Whereupon, the displacement quantity of the blade by the pressing is read by a non-contact type eddy current displacement meter and outputted to the control apparatus 12 through a displacement meter amplifier 11. A reference range of displacement quantity is preset to the control apparatus by calculating the displacement quantity to certain air pressure and the comparing operation of the actually measured displacement quantity outputted from the displacement meter amplifier 11 with said reference range is performed. As the result of the comparing operation, when the displacement quantity is within the reference range, the continuation of the operation of a machine is judged. When said displacement quantity is shifted from the reference range, the stop of the operation of the machine is ordered. Operation is stopped to adjust the tension of the inner peripheral blade so as to allow the deformation quantity of the blade at the time of the application of pressure to fall within the reference range.