PURPOSE:To suppress the lowering of sensitivity regardless of variations in a gap from an object, by winding two coils on a central magnet and magnets on both sides thereof each with a specified width to supply a required high frequency current. CONSTITUTION:Magnets 11b1, 11b2, 11c1 and 11c2 and the like with the width thereof half the magnet 11a are provided on both sides of the central magnet 11a with the width equal to the wavelength of an ultrasonic wave in such a manner as to differ in the polarity sequentially. The magnets 11b1, 11b2, 11c1 and 11c2 are wound with coils 10a and 10b arranged passing through the boundary and the center and current in the same direction flows through each of the magnets 11b1, 112, 11c1 and 11c2 to prevent the lowering of eddy current due to the cancelling of the magnetic field. Moreover, a high frequency current is fed to the coils 10a and 10b separately so that the amplitude ratio thereof will equal the amplitude ratio between the vibrations in the horizontal and vertical ways of the surface wave of an ultrasonic wave. This can reduce variations is the sensitivity regardless of changes in the gap (d) from an object 6.
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