A determination device for determining an improvement in water quality, capable of determining it in no time, is provided. The device has a first measuring device for measuring by AC terminal electric conductimetry a capacitance of a first capacitor formed between a pair of electrodes immersed in untreated water to be supplied to a water quality-improving apparatus; a second measuring device for measuring by AC terminal electric conductimetry a capacitance of a second capacitor formed between a pair of electrodes immersed in water discharged by the apparatus; and a processor for calculating a ratio (Y/X) of a capacitance Y to a capacitance X wherein the capacitance X is a capacitance of the first capacitor outputted by the first measuring device when an AC frequency is 100 Hz or less, and the capacitance Y is a capacitance of the second capacitor at the same frequency outputted by the second measuring device.