PURPOSE: To make the equipment lightweight without spoiling its rigidity and facilitate the portage by constituting an upper housing in an arch-shaped frame structure which is partially bored and combining and coupling the upper housing and a lower housing with each other with other members.

CONSTITUTION: The upper housing 1 is formed in an arch-shaped frame structure and plural holes 1a and 1b are formed on the top surface to reduce the weight. The rate of occupation of the holes 1a and 1b depends upon the width and thickness of the arch-shaped frame structure and can not be prescribed indiscriminately, but is preferably 25 to 30%. The decrease in the rigidity due to the formation of the holes 1a and 1b in the upper housing 1 is compensated by fixing the upper part of a reflecting mirror 6 to the upper housing 1 and the lower part of the reflecting mirror 6 to the lower housing 2 through a reflecting mirror fixture 7 with screws. Further, a rear cover 3 is molded of a plastic material with compressed air to form top-bottom integrated structure, whose periphery is fixed to the upper housing 1 and lower housing 2 with screws, thereby increasing the rigidity of the equipment after assembly.