PURPOSE: To measure a tension highly accurately without generating measuring errors, by vibrating a object to be measured by applying an external force, detecting a change of shifting amount of the object during a resonance/attenuating vibration after the external force is stopped, and obtained a resonant frequency.

CONSTITUTION: When a grid element 7a is vibrated from the outside and then the vibration is stopped, a resonance/attenuating motion of the grid element 7a is detected by a displacement sensor 53. More specifically, an analog output voltage waveform is input to an HPF 61 to cut direct current components and low frequency components. Thereafter, the waveform is amplified at an amplifier 62 and further high frequency components are cut at an LPF 63. The processed waveform is input to a waveform- converting part 64, where a sine wave is converted to a rectangular wave. Then, the rectangular wave is input to a cycle-measuring part 65 and a cycle (T) of the rectangular wave is measured. A value of the cycle is taken into a control part 66 and converted into a frequency (f) according to f=1/T/. A memory 67 stores distance data and data of measuring positions in the right and left directions of the grid element 7a which are different depending on the kind of pipes. The data are supplied to the control part 66 correspondingly to the kind of pipes.