Abstract: A shock sensor unit includes first and second sensor shafts having bulging portions, first and second positioning pins in contact with the bulging portions, first and second spring members that press the first and second sensor shafts, first and second sensor bodies that contain the first and second positioning pins movably, and first and second detectors that detect movements of first and second spring guides. Each of the first and second positioning pins is located at two positions that are point-symmetrical with respect to a center point between first and second electrodes.