An optical fiber sensor can be interposed between a bumper reinforcement and a load transmitting plate to extend in the direction of width of a vehicle along with the bumper reinforcement. In this case, when a change does not occur substantially in an immediately previous predetermined period in an output voltage of a light emission amount of the optical fiber sensor, a feedback control is performed in such a way that the change becomes a level of zero of the output voltage of the optical fiber sensor. In contrast, when a change in the output voltage is not substantially zero, the immediately previous state of emission of light is maintained. Accordingly, it is possible to effectively compensate a zero point shift caused by a temperature change or/and an aging degradation of the optical fiber sensor.