PURPOSE:To measure an interval by simple constitution regardless of the conductivity of an object to be measured, by applying the first and second high frequency voltages to the electrostatic capacity formed by an interval to be measured and performing an operation on the basis of a measured current value. CONSTITUTION:A flat electrode 10 is arranged in the vicinity of an object 12 to be measured having a dielectric membrane formed to the surface thereof and oscillators OS1, OS2 are connected to apply high frequency electric fields of frequencies omega1, omega2. Measuring devices 14, 20 are connected to the output side of an ammeter AM and respectively measure the amplitudes of the components of frequencies omega1, omega2 in the AC current detected by the ammeter AM. Herein, an operation circuit 22 performs operation on the basis of a predetermined formula and the interval (x) between the electrode 10 and the object 12 can be calculated. Since said predetermined formula contains no resistance value of the object 12, the interval (x) can be accurately measured regardless of the conductivity or material quality of the object 12.