To provide a holding/releasing mechanism of upper and lower cases housing an underwater developing object, which is reduced in number of parts, simple in structure and superior in operation reliability.

The truncated cone-shaped stopper (11) of the holding/releasing mechanism (30) is hooked with the hole (9) of a lower case (6) by the presence of the key pin (10) inserted in the hole (9) to hold upper and lower cases (5, 6). When an operation string (7) is pulled, to pull off the key pin (10) from the hole (9), the lower case (6) becomes free to permit content (12) to develop. When an underwater dropped sedimentation object equipped with this holding/ releasing mechanism reaches a prescribed depth, the key pin (10) is automatically pulled off to develop the content (12).