To remove sludge and to maintain dephosphorization with high efficiency for a long period.

This waste water treating device 1 is provided with a waste water treating room 4 having an inflow port 2 of the waste water and an outflow port 3, at least one pair of electrodes 5 arranged to face each other at least one portion of which is immersed into the waste water in the treating room 4 and incorporating iron and/or aluminum in at least one of them, and an electric power source with which power is supplied to the electrodes 5. An ultrasonic element 14 being a pulverizing mechanism for agitating and pulverizing the sludge depositing in the waste water treating room 4, and a control 15 are provided in the device.