PURPOSE: To surely separate a buoy for underwater observation while providing simple constitution.

CONSTITUTION: As a base end part of a hook 51 is pressed and fixed on a mounting base 4 by means of an electric corrosion wire 52, the hook 51 is held in a fallen condition. Under this condition, the tip part of the hook 51 is inserted into a mounting hole 21 arranged in a buoy 2 for underwater observation. As a result, the buoy 2 for underwater observation is fixed in the hook 51 so as to be moored on the mounting base 4. On the other hand, when electric current flows in the electric corrosion wire 52 on the basis of a command from the underwater observation device 1, the wire 52 is fused, and the hook 51 is rotated around a shaft by energizing force by a spring 57 and is raised from the fallen condition. In this process, the hook 51 is released from the buoy 2 for underwater observation.