Abstract: A vibration sensor has a bell shaped casing with a recess formed at its closed end. The recess has a flat bottom and slanted side walls so that an opening of the recess is larger than the bottom of the recess. The casing serves as a first electrode. Mercury of drop-shape is disposed in the recess, and at least two second electrodes are disposed circumferentially around the mercury. This structure creates three vibration systems; i.e., the mercury vibrates in three different modes. The first vibration system is based on the surface tension of the mercury. A portion of the mercury protruding from the recess undergoes vibration. The second vibration system is due to gravity acting on the mercury. The mercury rolls out of the recess between the electrodes in response to vibrations. The third vibration system is based on resonance. The mercury resonates between the electrodes.