PURPOSE: To reduce the probability that radiant rays are simultaneously penetrated into corresponding circuit elements of both printed boards by arranging the corresponding circuit elements at positions symmetrical about respective center points of both printed boards.

CONSTITUTION: Both printed boards 41, 42 have quite the same print pattern. The same circuit elements 43a, 43b, 43c, 43d are fitted to one side of the printed boards 41, 42 at the same positions and connectors 44, 45 are fitted to the other side of the printed boards 41, 42 so as to be opposed each other. The same circuit elements of respective printed boards 41, 42 are arranged symmetrical each other about the center line along the longitudinal direction of the printed boards 41, 42, so that the probability to penetrate simultaneously radiant rays into the same circuit elements is reduced.