PURPOSE: To let a spring be readily deflected and make possible easy operation at the time of assembling and disassembling without impairing the fixing strength of case by providing the spring to the portion having no relation with the strength of the portion for fixing case parts such as of outer case and inner case and using a weak spring.

CONSTITUTION: A fixing groove 103 in which a biting spring 104 is disposed is provided to an inner case 102 and a stopping part 101a for stopping the engaging part 104a of the spring 104 and the stopper 101b for stopping the movement of the spring 104 are provided in opposition to the fixing groove 103 by making a hole 101c of an outer case 101. At the time of assembling, a packing 106, windshield 105 and case 101 are put on the inner case 102, then the outer case end face 104d contacts the sloped face of the engaging part 104a inclinedly projecting from the groove 103. The engaging part retracts and when the end of the hole 101c passes the end face of the engaging part, the engaging part enters the hole 101c and braces between the engaging part and groove 103, and is located by the stopper 101b. Hence, at the time of separating the outer case and inner case, these may be readily separted by the stability of the packing from the stopping part simply by inserting a pincette or the like through the hole and pushing the engaging part