PURPOSE: To hold high damp proof and electric insulation over a long period of time by surely isolating the whole surface of a strain gage affixed to a strain-caused part and an object to be measured and the surface of the affixed part of the strain-caused part, on which the strain gage is affixed, from the outside air.

CONSTITUTION: A strain gage 1 on which a laminate layer 10 is made on a surface and a relay terminal 6 are affixed on the predetermined part of a strain- caused part 5 and a coating layer 9 composed of organic material is formed on the surface of each member including the strain gage 1, gage leads 7, the relay terminal 6 and the ends of lead wires 8. Thereafter, conditioning treatment of electroless plating is conducted on the whole surface of the coating layer 9 and the surface of the affixed part of the strain-caused part, after a washing process, activating treatment is performed, for instance, Ni contained in activating treatment liquid is deposited on the coating layer and the whole surface of the affixed part and a metal plating layer is formed. Thereby damp proof of the strain gage 1 is remarkably improved and the detection performance of the strain gage 1 is highly precisely maintained over a long period of time.