To sort out the earth and sand flowing into a sand basin according to the particle sizes and materialize efficient discharge of the sand through a sand discharging pipe.

The sand basin 1 includes a bottom structure 20 having larger depths toward the lower stream of a flow passage 4, and sand discharging pipes 7 respectively installed on predetermined different depth positions of the bottom structure 20. The sand discharging pipes 7 respectively have openings 5 having larger diameters as the positions get shallower.