PURPOSE: To detect distortion of a structure to prevent fatigue failure by mounting sensor main bodies, each made of a notched metallic piece, and their specific resistances in a plurality of monitoring positions, and connecting sensors in parallel with one another, and specifying the broken part of the sensors from change in the total resistance of this circuit.

CONSTITUTION: Sensor main bodies, each made of a rectangular thin steel plate with a notch at its center portion, are affixed to a structure 5 with an insulating adhesive 4, their specific resistances 8-1-8-n are connected to a lead wire 3, and sensors 1-1 are connected in parallel with one another, to constitute a resistance circuit. When the specific resistances of the sensors are R1-Rn, the resistance value of the overall circuit 1/R=1/R1+...1/Rn. When the sensor main body 1 having the specific resistance R1 is broken by the distortion of the structure 5, for example, the resistance value 1/R of the overall circuit varies and the broken part (the sensor having the specific resistance R1) can be recognized from the rate of this variation; that is, a data processing unit 7 monitors voltage fluctuations at both ends of a monitoring resistance 6, thereby specifying the broken part to be monitored of the metallic piece.