elastic member such as a rubber the elastic member is formed of multiple elastic portions that are disposed in the vehicle width direction with given intervals therefore the current conducting lines 11 12 face directly each other in spaces formed between the adjoining elastic portions when a collision is generated on the current conducting line 11 the current conducting line 11 is biased to move rearward and compresses the elastic portions the current conducting line 11 thereby makes direct contact with the current conduction line 12 further when the collision is then removed compression of the rubber is released and the current conducting line 11 returns to the original position to be reused the contact between the current conducting lines 11 12 requires a threshold collision load more than a given level to prevent mis detection in fig1 a circuit can be exchangeable between the two current conducting lines 11 12 in this embodiment the current conducting line 11 has a negligible low resistance while the current conducting line 12 or resistance line has a given level resistance here both can be exchangeable with each other the current conducting line 11 is grounded by contrast power source voltage vc is applied to the current conducting line 12 via resistances r 1 r 2 that are connected with the both ends of the resistance line 12 as shown in fig1 here the power source voltage vc can be applied to the current conducting line 11 while the current conducting line 12 can be grounded thus when a collision is not generated voltages vo 1 vo 2 at connecting points of the resistances r 1 r 2.