PROBLEM TO BE SOLVED: To enhance accuracy in measurement of a tension by calculating wave propagation speeds of vibrations of a specific frequency measured at measurement points set on an electric wire via a constant distance, and calculating the tension of the electric wire from an average speed.
SOLUTION: An accelerometer 2 is set at two points of a train line 1 separated by a constant distance L. A vibration-measuring means is constituted of the accelerometer 2 and a strain amplifier 3. A vibration acceleration of residual vibrations generated at the train line 1 when the train passes is measured. The vibrations may be generated by applying a shock artificially. A measuring apparatus 20 is provided with an A/D converter and a memory 4 as a data- recording means, a microcomputer 5 as an operating means, a controller and a display device 6 in addition to the strain amplifier 3. The data-recording means A/D converts measured vibration waveforms of approximately 5 seconds and records in the memory. The data are processed by the operating means, whereby a wave propagation speed is calculated with the use of 100Hz or lower frequency. A tension is calculated with the use of an average value of the wave propagation speed.