An apparatus for continuous dissolution comprising a dissolution portion for dissolving a gas into a main stream liquid, which comprises a flow meter measuring the flow rate of the main stream liquid and outputting a signal of the value obtained by the measurement and a mechanism for controlling the flow rate which controls the amount of supply of the gas based on the input signal; and a process for continuously dissolving a gas into the main stream liquid, wherein the amount of the gas is controlled based on the flow rate of the main stream liquid. Since a solution having a constant concentration can be obtained with stability even when the flow rate of the main stream liquid changes, cleaning water or surface treatment water used for electronic materials which particularly require a precisely clean surface can be supplied without loss.