PURPOSE: To automatically set the depth of a buoy on the basis of the detection signal supplied from a water pressure sensor installed on the buoy, as a submersible buoy device used for a hydrophone, in-water observation device, etc.

CONSTITUTION: A submersible body device is equipped with a buoy 1, mooring cable 2 for the buoy 1, and a weight 3 which is connected with the lower edge of the mooring cable 2 and set on the sea bottom, and further equipped with a mooring cable storage part 6a which can adjust the driving-out length of the mooring cable 2, in order to hold the buoy 1 at a necessary level in the water, brake 6b which can suspend the driving-out length of the mooring cable 2 supplied from a mooring cable storage part, water pressure sensor 4 installed on the buoy 1, and a controller 5 which controls the operation of the brake 6b on the basis of the detection signal supplied from the water pressure sensor 4. Accordingly, the buoy 1 can be automatically set at a necessary level in the water.