A method of controlling the concentration of P of cleaning solution waste water starts by providing a waste water buffer tank capable of either sending the waste water to a waste water treatment plant or storing the waste water after the waste water is accepted. The concentration of P in the cleaning solution is set within a target concentration range, and matter to be treated is treated. The concentration of P in the waste water is measured in the waste water buffer tank. When the measured value of the concentration of P is less than a selected upper limit, the waste water is sent from the waste water buffer tank to the waste water treatment plant. When the measured value of the concentration of P is higher than the selected upper limit, sending the waste water to the waste water treatment plant is stopped, and the waste water is stored in the waste water buffer tank. The concentration of P is reduced to a value within the target concentration range, and the matter to be treated is treated.