PURPOSE: To exclude the effect of external magnetism, by fixing either of a light emitting device or a light receiving device to a device cabinet, inhibiting the movement of the other device with springs and movably holding it.

CONSTITUTION: The cabinet 1 of the vibration detector 1 is constituted by combining a pair of tubular bodies 2 and 3 having bottoms so as to form a dark room 4 in the inside. The movement of the light emitting device 7 is suppressed by the spring 9 and movably held at the center of one box 2. Meanwhile, the light receiving device 11 is fixed at the center of the other box 3 by an attaching plate 12. The optical axes of said light emitting device 7 and light receiving device 11 are agreed under the normal stage. When the light emitting device 7 is moved by the vibration, shocks, or the like the directions of both devices are changed. Normally, a driving circuit 13 is opearted, the light emitting device 7 is turned ON, and the light hits the light receiving device 11. A receiving circuit 14 is in the monitoring state and a switching circuit 15 is in the OFF state. When the light is not received due to the vibration, shocks, or the like, the receiving circuit 14 detects the vibration, shocks, or the like, and the switching circuit 15 turned ON.