PURPOSE: To prevent the deterioration of the visual appearance of the title apparatus due to a scattering of a bonding agent by a method wherein L-shaped protrusions are respectively provided toward inward and outward on the side surfaces of the mounting surface of a decorative sheet and on both sides of the longitudinal direction of the mounting surface, an L-shaped protrusion is provided at the central part in the longitudinal direction of this mounting surface and the L-shaped protrusions provided on the above mounting surface are inserted in engaging holes, which correspond to these protrusions provided at three places, in a case and are engaged with the holes.

CONSTITUTION: Protrusions 4d provided on a decorative sheet 4 are inserted in engaging holes 5d, then, the sheet 4 is made to curve into the shape of a segment and protrusions 4b are inserted in engaging holes 5b and are made to engage with the holes 5b. Then, in order to insert protrusions 4c turned in the direction identical with that of the protrusions 4b in engaging holes 5c, a force goes being made to slide while the sheet 4 is pushed from the tip part 4f of the sheet 4 provided with the protrusions 4b to the direction of the protrusions 4c. Whereupon, the tips of the protrusions 4c situated in the intermediate position between the protrusions 4d and 4b graze the corner parts of the holes 5c, are made to advance to the interiors of the holes 5c and the insertion and fitting of the protrusions are finally completed. By this constitution, 3 pieces of L-shaped protrusions formed on the sheet 4 are respectively inserted in engaging holes, which correspond to the L-shaped protrusions, in a case and are made to engage with the engaging holes. Thereby, a mounting of the sheet 4 on the case can be performed.