The vibration rod of VL12 is vibrated by using the electro magnet and the permanent magnet which are faced at the right angles. When the electromagnet is energized, the electro magnet and the permanent magnet are attracted and repulsed. This movement makes vibration. When the vibration rod is covered with solids or powdered material, thecurrent feeding to the electro magnet is increased by damping of the back electromotive current. This change is detected and converted to output signal.