source is connected the sprocket roller 19 and the sprocket roller 37 are driven forcibly by the chain 52 accordingly the sprocket 19 engages the meshed member 11 of the lower belt 10 and drives the lower belt 10 in the direction of arrow b in the same way the sprocket roller 37 engages the meshed member 31 at both edges of the upper belt 30 and drives the upper belt 30 in the direction of arrow c at the same speed as the lower belt 10 is driven the concentrated sludge or scum a scraped by the skimmer 26 of the flotation device 25 is received in the scum chute 24 and with the movement of the lower belt in the direction of arrow b sludge is supplied from the discharge port 24b onto the filter sheet 13 of the lower belt 10 as both sides of the discharge port 24b are covered with the skirts 27a and 27b sludge is drained onto the lower belt 10 without leaking extending to the width edges of the lower belt 10 with movement of the lower belt 10 concentrated sludge a is supplied constantly on the lower belt 10 and proceeds to the press drum 38.