To provide a water flow distributing plate which is a water flow conversion member suitable for uniformly distributing an upflow in the peripheral direction and a flat water area cleaning device provided with this water flow dispersing plate.

In this flat water area cleaning device wherein from below an underwater hollow cylinder 4a, submerged in a pond 10 or the like or in a cleaning tank 1, gaseous ozone 12 is discharged to produce an upflow 112 along the underwater cylinder 4a and form convection on the periphery of the underwater cylinder 4a, a water flow distributing plate arranged above the underwater cylinder 4a so that the upflow 112 hits against it and for converting the upflow 112 into a distributed flow uniformly distributed around the underwater cylinder 4a.