Abstract: A theft preventing device comprises: a lock unit 11; and a coupling member 12 having both terminals 12a and 12b which are connected to the lock unit 11, at least one of the terminals being separable from the lock unit. In the device, the inside of the coupling member 12 is filled with a fluid including adhesive material, so that, when the coupling member 12 is damaged, the fluid is splashed out of the coupling member 12 whereby the adhesive material is stuck onto an object or objects around the device. Hence, from detection of the adhesive material, it can be determined who has broken it, and it can be confirmed that the lock unit has been locked.