PURPOSE: To prevent the generation of electrical arc in an interface between a metal fitting device and a synthetic structure by forming a void-less interface between the metal fitting device and the synthetic structure.

CONSTITUTION: A fuel tank wall surface 16 has multiple graphite fibers 32 having an intermediate diameter and formed with a fabric layer to be filled with the resin 34. The wall surface 16 is formed with an opening 18, and the opening 18 is formed with an inner screw 36. A cooperative screw 40 is formed in a narrow and long member 22 of a fuel temperature sensor 14. The sensor 14 is continuously rotated for insertion into the opening 18 by using a high-speed driver 50 having a constant torque, and temperature is raised by the friction between the screws 36, 40. When the rotation thereof is stopped before the sensor 14 is perfectly installed, the sensor 14 is fixed in the opening 18. An interface between the sensor 14 and the wall 16 is previously adjusted by the heat generation so as to eliminate voids. Only the fiber 32 having the mean diameter or more contacts with the screw 40, and they function as an electrical conductor so as to reduce the resistance heat.