PURPOSE: To accurately measure a tuning frequency with a simple and an inexpensive constitution by producing a saw-tooth voltage waveform from the same sweeping signal as one applying to a coil.

CONSTITUTION: The sweeping signal outputted from a sweeping oscillator 1 is applied to the coil 6 to be measured wherein a capacitor 4 and a coil 5 are connected in parallel after the signal is subjected to a constant current through an amplifier 2 and a resistor 3. This signal is amplified 7, then rectified by a diode 8 and a capacitor 9, and applied to a pulse generator 11. In the pulse generator 11, a pulse (d) is generated with a timing corresponding to the peak resonance point of the coil 6. The sweeping signal of oscillator 1 is supplied also to the amplifier 12 simultaneously and sent to a frequency/voltage converter 14 through a frequency divider 13. This sweeping signal is converted to a voltage proportional to the frequency and made to be the saw-tooth waveform, then sampled by the pulse (d) in a sample-hold circuit 15 wherein the voltage at that time is held. This voltage is arithmetically processed by a CPU 17 after an A/D conversion 16 is executed.