A determination device for determining antirust effect of treated water comprising a first potential difference measurement device and a second potential difference measurement device, wherein the first potential difference measurement device includes: at least one anode electrode (2; 12, 22), a pair of cathode electrodes having a first cathode electrode and a second cathode electrode, a first current generator for applying a first electric current between the at least one anode electrode (2; 12) and the first cathode electrode, a second current generator for applying a second electric current between the at least one anode electrode (2; 22) and the second cathode electrode, an electric current changer for periodically changing a magnitude of an electric current between the first cathode electrode and the second cathode electrode, and a measurement and output device for measuring a first potential difference across the first cathode electrode and the second cathode electrode, wherein the second potential difference measurement device includes: at least one comparative anode electrode (52; 62, 72), a pair of comparative cathode electrodes having a first comparative cathode electrode and a second comparative cathode electrode, a first current generator for comparison, for applying a first electric current for comparison between the at least one comparative anode electrode (52; 62) and the first comparative cathode electrode, a second current generator for comparison, for applying a second electric current for comparison between the at least one comparative anode electrode (52; 72) and the second comparative cathode electrode, an electric current changer for comparison, for periodically changing magnitude of an electric current between the first comparative cathode electrode and the second comparative cathode electrode, and a measurement and output device for comparison, for measuring a second potential difference across the first comparative cathode electrode and the second comparative cathode electrode, wherein the at least one anode electrode (2; 12, 22) and the pair of cathode electrodes (3; 13, 23) of the first potential difference measurement device are immersed, in use, in treated water that has been treated with a water treatment device, and the at least one comparative anode electrode (52; 62, 72) and the pair of comparative cathode electrodes (53; 63, 73) of the second potential difference measurement device are immersed, in use, in untreated water that has not been treated with a water treatment device, and a determiner for determining antirust effect of the treated water based on an output signal A outputted by the measurement and output device of the first potential difference measurement device and an output signal B outputted by the measurement and output device for comparison of the secondpotential difference measurement device. at least one anode electrode (2; 12, 22), a pair of cathode electrodes having a first cathode electrode and a second cathode electrode, a first current generator for applying a first electric current between the at least one anode electrode (2; 12) and the first cathode electrode, a second current generator for applying a second electric current between the at least one anode electrode (2; 22) and the second cathode electrode, an electric current changer for periodically changing a magnitude of an electric current between the first cathode electrode and the second cathode electrode, and a measurement and output device for measuring a first potential difference across the first cathode electrode and the second cathode electrode, at least one comparative anode electrode (52; 62, 72), a pair of comparative cathode electrodes having a first comparative cathode electrode and a second comparative cathode electrode, a first current generator for comparison, for applying a first electric current for comparison between the at least one comparative anode electrode (52; 62) and the first comparative cathode electrode, a second current generator for comparison, for applying a second electric current for comparison between the at least one comparative anode electrode (52; 72) and the second comparative cathode electrode, an electric current changer for comparison, for periodically changing magnitude of an electric current between the first comparative cathode electrode and the second comparative cathode electrode, and a measurement and output device for comparison, for measuring a second potential difference across the first comparative cathode electrode and the second comparative cathode electrode, wherein the at least one anode electrode (2; 12, 22) and the pair of cathode electrodes (3; 13, 23) of the first potential difference measurement device are immersed, in use, in treated water that has been treated with a water treatment device, and the at least one comparative anode electrode (52; 62, 72) and the pair of comparative cathode electrodes (53; 63, 73) of the second potential difference measurement device are immersed, in use, in untreated water that has not been treated with a water treatment device, and a determiner for determining antirust effect of the treated water based on an output signal A outputted by the measurement and output device of the first potential difference measurement device and an output signal B outputted by the measurement and output device for comparison of the secondpotential difference measurement device.