To propagate an energy efficiently in a required direction and to measure a distance between long-distance wells accurately even if a casing with a large energy loss is used by pressing an anvil for contacting wall surface being provided on the probing hole side wall of a vibrator in an oscillator to a probing hole wall with a clamping mechanism.

A pair of oscillators 32 is fixed into a well 12 by a hydraulic clamping mechanism 64 with a hydraulic source 68. The oscillator 32 accommodates a piezoelectric element lamination body 34 with axes in a plurality of horizontal directions and is arranged in the axial direction of the well 12. For example, a stainless anvil 42 is provided in front of a piezoelectric element lamination body 34. Then, one edge of the piezoelectric element lamination body 34 is pressed and fixed to a casing via the anvil 42 using a hydraulic piston, thus synthesizing vibration being generated due to the piezoelectric element lamination body 34 and propagating a sufficient energy efficiently to a ground layer even with a low frequency that cannot be outputted by a single piezoelectric element lamination body.