PURPOSE: To prevent occurrence of deformation or damage of a metal C ring and to enable simple and efficient execution of temporary assembly by providing a spacer and a guide pin on the occasion of the temporary assembly of a vessel and a cover in an optical submarine relay or the like.

CONSTITUTION: A spacer 3 which is held by the end part of a vessel 1 and the joint surface of a cover 2 between and has a thickness large enough to prevent deformation of a metal C ring 8, and a guide pin 6 which is constructed of a fore end part being fit to a threaded hole 4 in the vessel 1 and a rod-shape part to be fitted in a screw hole 5 of the cover 2 and guides the spacer 3 and the cover 2 in their insertion, are provided. On the occasion of temporary assembly of the cover 2 with the vessel 1, the guide pin 6 is fitted to the threaded hole 4 of the vessel 1 through the intermediary of the spacer 3, and then the cover 2 is assembled temporarily with the vessel 1 while the guide pin 6 is inserted into the screw hole 5 of the cover. The temporary assembly is performed by screwing lightly by utilizing two or three female screws 4 out of ones provided in a plurality on the circumference of the end face of the vessel 1. Thereafter, measurement of a loss of a connecting spot 18 of optical fibers is conducted, and after the measurement is completed, the cover 2 is fastened and fixed completely to the vessel 1 by screws 9.