To achieve efficient separation of water from sludge, efficiency improvement of solar drying, and reduction of a time for solar drying.

A filtration separation tank 1 which includes a frame forming a space therein, a tank body 5 having a large number of through-holes formed in the frame, and a filter cloth 6 stuck to the tank body 5 so as to close the through-holes is prepared and installed on laid sand 11 in a solar drying bed 10. The filtration separation tank 1 is disposed so that sludge 12 may be located outside the filtration separation tank 1, whereby the sludge located outside the filtration separation tank 1 is filtered by the filter cloth 6 to collect filtered water in the filtration separation tank 1. The filtered water is discharged from drain holes 8 to be penetrated into the laid sand 11.