A circuit board having an impact sensor element thereon is mounted on a depressed portion formed between a pair of thick portions of a supporting member. The supporting member includes a pair of protecting walls between which a connector leading out electrical signals from the circuit board is disposed. The supporting member is fixed to a support frame of a vehicle by screws or bolts inserted through the thick portions. The supporting member is formed with a resin material as a unitary body. Since the circuit board and the connector are mounted on the supporting member at positions protected by solid structures of the supporting member, they are prevented from being hit by vehicle parts falling apart when a collision occurs.