PURPOSE: To improve the grounding characteristic of a substrate and to prevent the substrate from being cracked by fixing the dielectric substrate on a base metallic plate while using bolt via an elastic protection plate and a ring.

CONSTITUTION: The dielectric substrate 11 has a mounting hole 11a made of a ceramic and a circuit conductor formed by metallizing and electronic components are mounted on the surface and a ground conductor by metallizing is formed on the entire rear face. Captions 12, 13 are respectively a chassis and an elastic protection plate having a mounting hole 13a, around which a recess 13b is provided to arrange an elastic ring 14. A bolt 14 is inserted to respective mounting holes 13a, 11a of the protection plate 13, the substrate 11 and the ring 14, they are fixed to the chassis 12.