PURPOSE: To prevent wrong reaction to vertical vibration and cause appropriate reaction to horizontal vibration which is the second phare of earthquake, by attaching a lid having a plane on its central part, to the upper end of a bored cylindrical housing.

CONSTITUTION: When vertical vibration which is the initial phase of earthquake is applied or vertical vibration from an engine is applied, the plane of the lid 36 hinders the upward motion of a ball 32. The distance between the plane of the lid 36 and the ball 32 is kept so small that the ball does not go off a seat 35. Since the the plane of the inside of the lid 36 extends to the top of the housing 31 at an angle equal to the sum of the inclination angle θ of a slope 31a and an angle (d), the ball 32 rolls on the slope and goes off the seat 35 with an acceleration value dependent on the mass of the ball, the gradient of the slope, etc. to move up a detection arm 34 when horizonal vibration which is the second phase of earthquake is applied. This results in detecting the vibration of set acceleration.