To provide a wastewater treatment device for a road cutter which can be mounted on a vehicle, or the like, and can efficiently reuse suspended wastewater after the use of the road cutter without polluting the environment.

The wastewater treatment device 10 for the road cutter is constituted so as to recover suspended wastewater for use in lubrication and cooling, which is supplied to the edge of the road cutter 11 for cutting the road surface of a concrete road, or the like, to purify the recovered wastewater before circulating the same to the road cutter 11. The wastewater treatment deivce 10 has a suspended particle flocculating part 21 equipped with a flocculation flow channel wherein suspended wastewater is supplied to a gap between an anode plate 41 and a cathode plate 42 both of which are arranged parallelly, a filter part 23 equipped with an almost flat plate-shaped filter 22 which is formed into a bag shape as a whole, receiving suspended wastewater passed through the flocculating part 21 from its external surface side and discharging filtered treated water from its internal back surface side, and a treated water tank 10a incorporating the flocculating part 21 and the filter part 23.