To provide a small seismic sensor of small power consumption with simple constitution capable of correctly detecting both the vertical and horizontal vibrations of earthquake respectively.

The seismic sensor is constituted of: a horizontal vibration sensor unit 2 for detecting the horizontal vibration of the earthquake by conducting between a conductive pedestal 7 and contact body 8 through a conductive sphere 5 moving on the pedestal 7; a vertical vibration sensor unit 3 for detecting the vertical vibration of the earthquake by conducting between an electrode 18 and contacts 20, 21 through the moving piece 17 moving freely up and down along the axis 16 and the contacts 20, 21; and an output means 4 for informing the earthquake information based on that a logic sum outputs of the conduction signal of both the sensor units 2, 3 are derived, and after that the logic sum outputs are again derived after a previously set first time has elapsed, and during a second time.