PURPOSE: To strengthen protection of circuit elements to be miniaturized and mounted by inserting circuit elements into through holes of the second circuit board to join the first circuit board for mounting circuit elements on arbitrary positions on the front and rear faces and by coating circuit elements in the through holes of the second and third circuit boards with sealing resin.

CONSTITUTION: The first circuit board 21 is provided with the second and third circuit boards 22, 23 to be joined to the front and rear faces, respectively; and the second and third circuit boards are pierced by through holes 220, 230 to insert the respective circuit elements 12, 14 through at the areas of mounting circuit elements onto the first circuit board. The second and third circuit boards are joined to the front and rear faces of the first circuit board 21 to mount its elements 12, 14 to the through holes 220, 230 of the second and third circuit boards. The through holes are filled with sealing resin 20, which coats the circuit elements 12, 14. This process allows the circuit elements 12, 14 to be mounted into and protected by the board formed by the first, second, and third circuit boards and a circuit device to be miniaturized.