PURPOSE: To conduct the installation work of a bucket to a tank body efficiently in a short period of time by providing along with a tank body a valve mechanism for opening and closing a penetration hole made at the bottom of a bucket constituting a soil and sand separation tank device.

CONSTITUTION: A valve mechanism 19 comprising a valve plate 19b turning about a hinge 19a, a trimming member 19c, a heavy bob 19d, and a gate-shaped stopper 19e for limiting the open and close angle α of the valve plate 19b in order to open and close a penetration hole 10d made at the bottom plate 10c. When the bucket 19b is filled with soil and sand, the bucket 10 is lifted up by a lifting means to carry out discharge to a truck or the like, and when installed in the tank body again, the valve mechanism 19 is released so as to take water in the body into the bucket 10b, thereby executing prompt dropping and setting even in the case of water filled therein.