To provide a wafer treater, which can control accurately the descending speed of the level of pure water in a liquid discharge and drying treatment.

Pure water in a treating tank 1 is discharged through inner-tank liquid discharge pipes 109. During this discharge, an introducing tube 24 continues to feed IPA (isopropanol) vapor. As the liquid level of the pure water lowers by this discharge, a layer of IPA condensed on the liquid level of the pure water is deposited on each wafer W and as a result, droplets adhering to the surface of each wafer W are replaced by the condensed IPA layer. A control mechanism 6 detects the position of the liquid level of the pure water lowered by the discharge by a liquid level sensor 61 in the tank 1 and a micromanometer 62. The lowering speed of the liquid level of the pure water is calculated in an arithmetic circuit 63 on the basis of the position, which is measured by the sensor 61 and the micromanometer 62, of the liquid level. An electropnenmatic regulator 64 converts this calculation result into a pilot air pressure. A pressure adjuster 65 adjusts the opening of a quick open valve 66 by this pilot air pressure to control the flow rate of the pure water, which is discharged through the liquid discharge pipes 109.