To change the dissolved gas concn. in water with a simple device by putting a positive electrode and a negative electrode in water, impressing a feeble DC voltage and forming a fine gas bubble of the dissolved gas low in buoyancy as compared with water surface tension from the positive electrode.

This device is applied to a device for generating the water for an aquarium fish, a drinking water purifier, etc. A cylindrical stainless-metal electrode 2 or the one having a clumnar curved surface to which a fine oxygen gas bubble is hardly adhered is used as the positive and negative electrodes to prevent the oxidizing dissolution of the electrodes and operated with a low voltage of ≤5V. When the gaseous oxygen dissolved in water is concentrated, the oxidizing dissolution of the electrodes and the dissolution of the vessel close to the water surface are caused. Accordingly, when a large quantity of fine gas bubbles are formed, a current is intermittently applied by the use of a timer. Further, although a crystal is deposited on the negative electrode, the voltage to be impressed on the electrodes is inverted to impress a positive voltage on the negative electrode, and the deposit is removed.