PURPOSE: To safely operate a net conveyor, by preventing the injection of water dreg slurry from a cut discharge port by attaching a water dreg guide duct with length not less than longer than the height of a gate of the cut discharge port to the water dreg cut discharge port of a feed box.

CONSTITUTION: In a device that water dreg slurry a fed onto a net conveyor 3 from a water dreg cut discharge port 2 of a feed box 1 is changed into crude dehydrated water dreg b while being dehydrated, converted into dehydrated product water dreg c at a discharge end 3a and discharged onto the nexy stage belt conveyor 4 while dehydrated water is collected to a drain gutter 5 under the conveyor 3 and discharged from a drain pipe 6, a water dreg guide duct A, which has a tapered port 7 consisting of gate height H lower than the cut discharge height h of said cut discharge port 2 and gate width W narrower than cut discharge width w and which length L is set to be length not less than gate height H, is mounted to said cut discharge port 2, thus preventing the injection of water dreg slurry a from the tapered port 7