It is an object of the invention to provide an effective technique for detecting the quality of water with high accuracy. A representative water treatment apparatus includes an aerobic treatment region that treats water aerobically, a downstream region in which the water aerobically treated in the aerobic treatment region flows, a water quality sensor that is submerged in the downstream region and detects the water quality and a sensor washing arrangement that washes the water quality sensor by supplying the water of the downstream region to the water quality sensor at a flow rate higher than water flowing around the water quality sensor. As a result, sludge generated due to aerobic treatment can be prevented from being deposited on the water quality sensor and/or deposited sludge can be removed, so that the accuracy of water quality detection of the water quality sensor can be enhanced.