Abstract: The vibration of a belt or the like detected by a microphone is shaped into rectangular waveforms. The period of each cycle of waveform is held by a latch (8) and sequentially stored in a period memory (12). In a cycle comparison unit (14), waveforms in an expected allowable range of variations are extracted by comparison with the fiducial period. The series of the extracted waveforms are stored in a memory (16). A selection unit (18) selects a representative value from the series of extracted waveforms. A frequency calculating unit (19) calculates the frequency in accordance with the representative value. A tension calculating unit (20) assigns the calculated frequency (characteristic frequency) to a given formula to work out the tension of the belt or the like. It may be possible to calculate the frequency directly from the latched period and then execute the processes to follow.