PURPOSE: To hold back the force of spring back by bending an insulation sheet so that it may be box-shaped and installing some device like hook on the side of the box and a hole into which the hook may be seized.

CONSTITUTION: An insulation cover 1 which comprises an insulation sheet bent in a box shape is mounted on a circuit substrate 7. The side 4 is provided with a projected part 5 and a fit-in section 6 which is inserted into a long hole 3 provided on the circuit substrate 7. Each side of the insulation sheet 1 tends to extend outward by the force of spring back, but the projected part 5 is hooked with the L-shaped hole 3 so that it may be further fitted-in, thereby preventing the extension of each side of the sheet. At the same time when external force is applied to the side 4 of the insulation sheet 1 inwardly, the side 4 is protected from the inward deformation induced by external force since the long hole 8 is installed to the circuit substrate 7 and the projected part 6 provided on the side 4 of the insulation sheet 1 is fitted into the hole 8. It is, therefore, possible to prevent the extension of each side.