A dilute solution production apparatus produces a dilute solution of a second liquid containing at least one of a conductivity-imparting substance and an oxidation-reduction potential regulating substance by adding the second liquid to a first liquid, and is provided with: a first pipe through which the first liquid flows; a pump for adding the second liquid into of the first pipe via a second pipe; a degassing pipe which extends from the pump; a water quality sensor which is composed of a conductivity meter, a resistivity meter or an oxidation-reduction potential meter; and a control device for opening a degassing valve when a water quality detection value in the water quality sensor varies by a predetermined value or more.