PURPOSE: To attain fine adjustment for obtaining neutral buoyancy by separately storing a buoyancy material and a weight in a machine, and also arranging each cover openable/closable for respectively releasing separately the buoyancy material and the weight to outside the machine, in a storage part thereof, so as to adjust underwater weight of a machine body in both positive and negative directions.

CONSTITUTION: In an unmanned diving machine 1, in its central part, a vertically inserted buoyancy adjusting device 2 is arranged. The buoyancy adjusting device 2, by a partition 3 arranged in this central part, is divided into a storage part 21 for storing a buoyancy material 4 in the upper part and a storage part 22 for storing a weight 5 in the lower part. Further in both upper/lower both ends of both the upper/ lower storage parts 21, 22, each cover 6, 7 is respectively arranged, to constitute the storage part openable/closable by remote control or the like utilizing, for instance, an optical fiber cable 8. By opening the upper cover 6 to release a required amount of the buoyancy material 4 to outside the machine, buoyancy is decreased to sink the unmanned diving machine 1. On the other hand, by opening the lower cover 7 to release a required amount of the weight 5 to outside the machine, buoyancy is increased to lift the unmanned diving machine 1.