A diluted solution production method of the present invention is a diluted solution production method of producing a diluted solution of a second liquid by adding the second liquid to a first liquid, the method including feeding the first liquid to a first pipe; and controlling pressure in a tank that stores the second liquid to add, through the second pipe that connects the tank to the first pipe, the second liquid to the first liquid in the first pipe. Adding the second liquid includes measuring a flow rate of the first liquid or the diluted solution that flows through the first pipe; measuring a component concentration of the diluted solution; and controlling the pressure in the tank, based on the measured values of the flow rate and the component concentration, so as to adjust the component concentration of the diluted solution to a specified value.