A pure water producing apparatus prevents fluctuations in the amount of produced pure water without requiring a heat exchanger. Raw water is filtered with an activated carbon filtration device, sent through a water supply pump to the reverse osmosis device, and demineralized. Demineralized water output from the reverse osmosis device contacts a water quality sensor including a water temperature sensor, is sent to an electrodeionization device, and subjected to electrodeionization. Water subjected to electrodeionization contacts a water quality sensor having a water temperature sensor, and is taken out as treated water. A sensing signal output from the water temperature sensor is input to a pump control circuit to control an amount of the water supplied from the pump so that an amount of water passing through the reverse osmosis device is maintained constant.